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Update to this:

I ran CrossFire on my (2) 6990's for a couple days. It worked well - but I had to run my fans up quite a bit to ~70% to keep the cards around 80*C. Even with my Antec 1200 case, these cards still get crazy hot! I decided that I wanted to trasnfer the hardware out and put my GTX285 back into my main computer, and build a dedicated mining rig instead.

I ended up going to MicroCenter on Sunday and picked up the following. It's a nice cheap build, but very powerful and has plenty of room for expansion. I went with quad-core on the CPU in case I decide to use this machine for a web/dev server.

MSI 890FXA-GD70 Motherboard - $169.99 after rebate
CORSAIR XMS 4GB (2 x 2GB) RAM - $44.99 after rebate
AMD PHENOM II X4 840 CPU (Quad-core 3.2GHz) $97.99

Total cost: $312.97


The motherboard has 5 PCI-E slots. I'm only using 2 of them right now, and could probably fit a 3rd 6990 if I really wanted to. If I had a cooler running card, I could probably stack 4 5830/5850's in there, without issue. I ended up getting 2x2GB for RAM so I can use the other DIMM in another rig if I decide to build another one. I put all of these components along with my Corsair 1200 PSU into an old Chieftec case that I had lying around. I also put in an old WD 250GB HDD, and a 3TB Hitachi HDD, since I can now use this as a network backup machine as well. I installed Ubuntu 11.04 x64 Desktop, and I'm running the miners via poclbm right now.

I'm getting around ~324 Mhash/s on each GPU core. In Windows 7 x64, I was able to get ~350 Mhash/s using GUIMiner, so perhaps there's some tweaking that needs to be done.



My new issue is heat. This old Chieftec case does have 5 fans, but they're all small 80mm ones. I have a couple extra 120mm 72cfm fans lying around, so I'm going to try and position those effectively. The case is running at home now with GPU cores @ 77c - 79c - 80c- 74.5c, fans set to 85%. It should be noted that my side panel door is also open, but not completely off. It's resting against the case and the opening is towards the rear.

I'm also thinking about going caseless, since it looks like it will help quite a bit in terms of heat dissipation. The MyOpenPC test bench also looks VERY nice, found here: http://www.amazon.com/MYOPENPC-Transparent-Acrylic-Stackable-Capability/dp/B004TZAU1Q/ref=sr_1_11?ie=UTF8&qid=1306839609&sr=8-11

Perhaps I could get a couple of those and stack them. (if I had multiple builds) Another option is to buy some 4U rackmount cases. I find the mining process to be very interesting, and would love to build 2-3 more machines using the same components that I picked up this weekend for cheap. I could then rackmount all the machines on a server rack at home, or even get some colospace in a datacenter. I'd have to make it worthwhile to go that far though.


Thoughts?

I have 5 HDD's on my system, and a i7 920 @ 4 GHz.

Right now with (1) 6990 card running both GPU's in GUIMiner, I'm getting ~70*C with fans set to 75%. I also have my case fans running at 100%, but my side panel is off. (Antec 1200) I have another machine lying around that I'm debating on turning into a dedicated mining rig, using these 6990 cards. I have the following hardware available:

Core2Duo E6600
Asus P5B-E Mobo
6 GB RAM
Extra HDD's
Full tower case (early-style Alienware lookalike)

I have to check the spec's of the extra Asus MOBO, but I'm hoping it will support (2) PCI-Express cards at x16. If so, I might just put the cards into that case and turn it into a dedicated mining rig. I had plans to use it for an ESX environment to learn VMware, but we'll see. If I do switch things over, I'll put my GTX285 back into my main desktop machine.

The Corsair AX1200 is a single rail PSU.

So it sounds like you're *not* running CrossFire, correct? Any reason as to why not? I'm using GUIMiner for now; should I switch to Phoenix?

I'm also mining on deepbit now but thought about going solo...

Update:

I managed to get GUIMiner to work on both cores on 1 card. Both cores combined put out around ~670 Mhash/s without any tweaking. My GTX285 put out ~62. 

It looks like I had some old nVidia drivers lingering around my system. I used Driver Sweeper to clear them out, and then removed and reinstalled the ATi driver.

I'll try adding the second card tomorrow.

On my Asus P6T board there are 3 PCI-E x16 slots. Two are blue, one is white. (bottom one is white)

I've tried all types of slot configurations with no success. Are there any BIOS options that perhaps I'm missing? (ACPI?) I unplugged the other card and tried mining on just one card - as soon as I hit "Start" on GUIMiner, the machine locked up and froze.

Installed both cards and the AX1200 tonight - however I'm facing a new issue. I was able to install ATI Catalyst software w/ the latest drivers and SDK 2.4.

However, the system won't get past the "Starting windows" screen during Windows 7 x64 bootup, with both cards plugged in. If I unplug both PCI-E power connectors from one of the cards (doesn't matter which one), then system will boot up fine and will *not* hang at the "starting windows" screen. I'm not CrossFire'd or anything like that yet, just 2 indivudal cards. Do I need a VGA dummy plug or something? Eventually I'd like to CrossFire, but regardless if the GPU is being used or not, I'd like to be able to boot into Windows with both cards powered on and plugged in.

The PSU is 1200 watts. Could extra HDD's be the issue? I have the following:

(1) 80GB X25-M Intel SSD
(4) Samsung HD204UI 2TB HDDs in ICH10R RAID10

5 HDD's
(2) Diamond 6990's
i7 920 (4.0ghz on Air, stock for now just for testing)
Asus P6T (vanilla)
CoolerMaster V8
12GB Corsair Dominator 1600
1200Watt Corsair PSU (AX1200)

Thoughts?

Cards should be arriving later today.

Installing this as well later tonight:

I am running a dual 6990 setup. It's drawing about 1.12kW of power, with spikes up to 1.18kW.
These suckers are hot, and hungry.
And hot.
And loud.
And hot.

I hope you have a 3-slot or 4-slot motherboard, because sandwiching two 6990's together with only 1/4 inch of room between would be a huge mistake. Even separated by an empty slot, and cooled by TWO 200mm 166CFM fans (one from front and one from side), they still run around 95ºC with the GPU fans at 70%.

Can't complain about getting 380MH/s x4, though.

Probably could overclock and get 400MH/s with watercooling.


I should also note that my power draw is higher than yours because I also use this as my database server. Four hard drives and an Intel i7 950 will do that.

If that PSU is not a single-rail system, then make sure to look up which modular slots correspond to which rail, and load them equally.

Also, without using dummy plugs, phoenix will only see the GPU that the monitor is currently plugged into at the moment; What I do is plug the monitor into my 5830 and run Phoenix/PhatK, then unplug it and swap to the 5850 and run the 5850.bat, then unplug the monitor alltogether to get a slight 2 MH/s increase (seen by logging in remotely via TightVNC)

2x Stock 6990's use roughly 700W combined.

The rest of your setup demands atleast 200W.

Your PSU *should* be able to handle that. Could be something wrong with your 2nd PCI-E port.

Try putting only 1 card in that slot into your motherboard, and see what happens.

Windows can be wierd about crossfire.  I have a 6990 and 2 5870 but no crossfire enabled, then I have another box with 2 5970 that requires the crossfire cable to be installed in order for opencl to recognize all 4 cores.  It used to be (stream 2.1) that crossfire degraded performance, but I've seen that with sdk 2.4 that problem has been fixed.

EP

This comparison is meaningless without Mobo/Ram/CPU/PSU/Case/HD costs factored in.  In my case (dual 5970 rigs) GPUs make up 65% of the cost of a mining rig.  For 5850s hardware costs will be over 50%

IMO 5850's are the best reference card, costing slightly over $100 dollars and yielding from 280 to 330 mhash/s.
In comparison a $500-$600 dual GPU card fetching merely ~650-700mhash/s at stock speeds seems like a bad hardware investment.

The only problem is availability, you would be hard pressed to find many in new or mint condition. However there are plenty of used ones on the open market.

1200 corsair PSU is great.  No such thing as too much capacity.  That will allow you to run 3 cards in the future

Windows for now until I decide on the volts, clock, and fan speed I want to run and then I'll modify the bios and run linux.  Afaik if you switch to the second bios it will disable power throttling but I'm running on a 520W PSU so don't want to risk this (my rig uses 360W at 300MH/s x2 (750MHz iirc)).

750W for two 6990 is not enough.  I have two 6990 running and they are using 880W while mining under full load (with a 1000W PSU).  Also, they throw off tons of heat and suck more power than my 4x9650 setup.  The 4x9650 draw under 800W under full mining load, run cooler and get a better hash rate than my 2x6990.  And the 6950s were way cheaper to buy. 

Hope it helps. 

Are you running Windows or linux with on your setup? I'm looking for more info on the power throttling under linux, but so far have only found anything useful for Windows.

Try running a single core on the 6990 and see if you get closer to 400MHash/s.  Also check to see if your core speeds stay at 880MHz or are jumping about a bit.  I found the power throttling kicked in on mine and needs to be raised before you can get 400MH/s on both cores at the same time.

I had a 650WCorsair PSU and could only run the card on 830MHz (but then again my CPU is a i7-2600. I recommend getting a stronger PSU if you want to run two cards, I got a Thermaltake 1200W for my system running 2 cards @ 880MHz.

On a side note, when people get 800MHash/s on the 6990, how do they go about that?
I average about 310-320MHash/s per 880MHz GPU core (running Ubuntu 10.04 x64, Catalyst 11.5, 2.4 drivers, and Phoenix 1.48). On my Windows 7 x64 box I get 343 MHash/sec on Phoenix 1.48 and the latest drivers/SDK. Suggestions?

Another option for those looking for a good deal, the XFX HD-695X-ZNFC can be had for $209 after rebate and a number of people also unlocked them, not reference cards and 1GB of RAM but for BTC it may be a great option.