Now I am thinking a 7x 750TI rig is possible, using the ASRock card you listed.
I also think a 600 watt PS would be enough? I calculate 420 watts (7 * 60 per card) + 100 (MB, CPU, etc), or around 520 watts load. A quality 600 watt such as
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028 should be enough I hope.
The motherboard you mention
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157460 is about double of the famed
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157471, but what are you going to do? I was trying to keep the ancillary costs to around $200, but now closer to $300.
How much system memory do you think a 7x 750TI rig will need? 4 gig enough or go with 8 gig?
So I am coming up with:
$ 300 - MB, CPU, Memory, PS
$1,120 - 7 x $160 GTX750ti cards
$ 30 - shipping
so around $1,450 for a 7x rig capable of anywhere between 1960 kHash (280*7) to 2100 kHash (300*7) at only 550 watts draw.
Payback will be a bit longer than usual, but I think the key here is the lower overall power draw. A similar hashrate 3 x R9 290x (
edit should have been 280x) system would draw nearly 820 watts.
Then comes the tweaking. If indeed these can get greater than 300 kHash each and if they can be under-volted for even less power draw, it would swing the balance even more in favor of NVidia.
Ok, based upon my original assumptions from a week ago (above) and now with two cards in hand I can recalculate and see how close those assumptions were.
System power draw, idle with two cards is drawing 45 watts from the wall. Running my two 750TI's at 290 khash and overclocking them is drawing a total of 215 watts from the wall (kill-a-watt reading).
So I have:
215 system (2 cards hashing)
- 45 system idle
= 170 card consumption when hashing
170/2 = 85 watts per card when hashing.
So if I scale this back up compare with my original projections, I get 7 cards * 85 watts + 45 watts system = 633 watts system draw. This will require a 750 watt PS to be safe, 650 might cut it but why chance it.
My initial results with hash rates of ~290 khash per card would end up making a total of 290*7=2030 khash, lets call it 2k to be conservative.
So figuring on the side of caution I come up with 2 Mhash for 650 watts, or 3.07 khash/watt. Looking at one of my 3x R9 280 rigs, it is getting 2,160 kHash (2.16 Mhash) for 820 watts, or 2.634 khash/watt.
I think getting to a consistent 300-320 would help tip the balance, but I am on the fence right now. Maybe the newer Maxwell cards will perform a bit better, and maybe some further optimizations can be achieved with cudaminer. I think I will finish building this rig out to 6 (hopefully 7) cards, but after which I will go back to the Radeon route for the time being, unless the dualminers would come down in price.