Check the post. You'd have to use a PCIE extender and then mod a PCIE1x slot. It's doable, but could be difficult.
A 5970 is rated at 300W while a 5850 is rated at 150W. So energy-wise, a 5970 would be 2.67mhash/W while a 5850 would be 2.33mhash/W.
So at if you wanted to pull 1000mhash/s with 5970's you'd have about 375W from the cards alone. With 5850's at 1000mhash/s you'd need 429W for the cards alone.
Using the mhash/$1 calculations the 5970's at 1000mhash/s would cost $751. For the 5850's it would cost $429.
The 5970 setup would save you almost 55W, but would initially cost $321 more for the hardware. Using 55W less, it would save 1.3KWh/day, or 481.8 KWh/year. Let's go with a rate of $0.15/KWh. You'd save $72.27 in electricity with your setup. It would take you 4.44 years to justify the energy savings. The hardware for your setup is about the same, maybe $50 less for a mobo with only 3 PCIE slots, but the PSU would be nearly the same.
Using fractional GPUs in this calculation, but the numbers are still valid.
Your numbers are off. 700 Mh/s on a 5970 with stock voltage is realistic, 350 on a 5850 isn't. Even less with 6 of them in the same box. Think 320 top and even. Overall, a 3*5970 would be at least 150-200 Mh/s faster than 6*5850.
My remark wasn't mainly about the 5970, but the 5870, with which the maths becomes way different. Consider how much cheaper it is to get 2 of those than a single 5970. They're not as power efficient than the 5970, but more than the 5850. To get 350 Mh/s out of a stock 5850, you need to run it at 900mhz core, that's a 175mhz overclock on stock voltage. To run a 5870 at 400 Mh/s, you need a 90Mhz overclock. You have to consider feasibility. I was considering monster overclocks for my first rig, but you shouldn't, it's not that simple.
You also have to consider 2 others things: the 5850 is a middle end card and has already a low resell value compared to the 5870 and 5970. Of all the price decaying that computer hardware undergoes, the high end parts are the ones that resist the test of time the best. When the better cards are coming out and they are outrageously better while consuming the same power, you will need to upgrade, so the resell value of your cards will matter in the total equation.
Lastly, this isn't much, but by running 3*5970s, you don't need a 6 slot mobo nor the PCI-E extensions.
Well 5870's are rated at 188W while 5850's are 150W. I have two 5870's atm, I can get them up to about 990MHz with a voltage of 1.2v individually. That 90MHz overclock required a voltage increase. There's not much efficiency difference between the two. 2.13mhash/w for 5870 and 2.33mhash/w for 5850.
And I stated in the my post that I was using high end overclocks. The title does say "monster", all of the overclocks are achievable with some effort. My 5870's can reach up to 1GHz, but I start getting diminishing returns on gains in frequency per voltage increase and I haven't been able to tweak them enough. Maybe this crowd isn't as used to overclocking as me, but I like to get the most out of my hardware. Considering the 5xxx series is considered safe by many to run at up 1.3v, these overclocks are doable. Yes the overclocks would increase power consumptions, but I'd have to argue that increasing mhash by increasing power consumption is a cheaper alternative than increasing mhash by buying hardware.
Resale value of all of the 5xxx series is going to drop. 5870's can be found on forums used for as low as $150 and 5850's for as low as maybe $100. Either way you're going to lose about the same amount with these cards. Sure the 5970 will have a better resale value since it's an extreme card, but I don't think it will be significant. Also take into account the wear and tear on running these cards 24/7 will have on their resale value (if you're honest).
And I mentioned the part about the 3 slot PCI-E mobo, take $50 off the cost of that setup for a cheaper mobo. Still leaves a significant gap.