I have been advised that the amps on your 12V rails are more important than total watts.
get a good quality 650 or 700 watt PSU with at least 50 amps on the 12V rails. I have this system:
2 gig ram
asus striker extreme
xfi extreme gamer
2x 8800 gts 320 OC
5 or more case fans
and of course the cpu fan, my PSU has 2 fans etc.
and my PSU:
silverstone s12 80+ 650 W is running it great: 52 amps on the 12V rails (or so they claim, i have not measured).
So go for quality and not quantity: I paid $180CDN for the PSU (almost a year ago).