Good on ya. Count it as a blessing, you'll save a pretty penny on this one. If you're fixing Marine Corps air assets building a PC is monkey work.
http://www.gpureview.com/show_cards.php ... 39&card2=#
here is some info on the hd 6950 cards, including important bits of info suchs as
1. max tdp (Thermal design power) = 200W times 2, = 400W Max for just your video cards.
Thermal design power - Wikipedia, the free encyclopedia
Note that compenent manufacturers and psu manufacturers use various testing and marketing tricks to make it difficult to compare products, so keep that in mind.
2. the list of the 40+ retail designs of the hd 6950
depending on how many other peripherals you need a 650w to 750w power supply to be safe.
here is a PSU requirement calculator:
eXtreme Power Supply Calculator
Note this is also just a guide....
note, PSU capacitors degrade over time lowering the PSU efficiency, that's why you buy more than you need today, for longevity, don't be penny wise and pound foolish. Definitely Get 80+ gold efficiency, ... optionally get modular cabling.
Cooling has gotten easier to talk about, go buy a corsair H100 liquid cooling kit. It's a closed kit, takes about 4 screws and 5 minutes extra work compared to adding a air cooling fan.
Make sure you buy a case that can fit the liquid cooling radiator comfortably, it's the only thing that takes pre planning.
simply google corsair h100 + your case make and model and find some reviews/pics/youtube vids of people who have done it already. Don't reinvent the wheel.
Get interior case air circulation, your 2 hd 6950s will produce lots of heat when stressed during gaming sessions. A couple of high quality case fans is all you need, cost about $15 a piece max.
That's about it.
Another option is to go to some PC enthusiast forums, find someone with a AMD 8 core system, and just copy his design
PS: IN before the Buy one 7970 instead of two 6950 comments.