I agree, A quality 700 - 800 would be more than enough. That would be the thing to keep in mind though, using a GTX 285 will need most of the AMPS. I would suggest Corsair, Enermax, or Antec Signature. Hope that helps.
i7 920 @4Ghz and GTX285 @optimal OC can't even chew over 460W (not wall) when benched, a good modern 550W that has passed 80Plus can sustain >90% load 24/7 no problem.
Occasionally Corsair 550VX can be found cheap. Antec Earthwatt 550W is another that can be found for good price.
>700W is really meant for a mild OC'd dual-GPU rig.
Even DICE benching a Q6600 @4.3Ghz and X2900XT (crazy card) @1.1Ghz back then I could do it with a TT Toughpower 600W (80Plus CWT platform).