If it's just 2 6850s and an overclock you can get by with a good 650W. Corsair 650TX would be one less expensive choice.
The two cards themselves will not pull more than 250W:http://forums.atomicmpc.com.au/index.php?showtopic=264
Add another 200W for the rest of the system and throw in another 200 for margin, and a good 650W is fine.
However, if that is your total budget you should probably not be looking at i7 950. That's more like an AMD budget. Perhaps a 6-core CPU.