Currently running an amd 3500+, 1gb ram, x800pro. My memory is starting to fail, and my computer is really hindering my raid performance, along with enjoyment level when on my pc (i.e. not being able to run itunes + wow at the same time without the songs skipping).
I agree with MadAdmiral. You should go check out the ATi cards. The 5870 costs just 30$ more and it gives a notable performance increase in most games. Even the HD5850 is better in a number of games, and it's about 70$ cheaper also. So I'd definitely check those out if I were you. Unfortunately there's a quite long wait time for those atm.
And that 1000W PSU is indeed overkill.
From what I've heard, you should be able to power a system with an overclocked intel core i7 920 and 2 HD5850's with a 750W corsair, for example. Well either a 750 or 850W PSU anyway.