Yeah, you'll want to watch out when buying a PSU for a number of reasons.
A lot of power supplies will label themselves as 400 watt, but this is really their peak output, meaning they may only put out a steady 350 watts of power and may peak for short periods at 400 watts. You'll want a power supply that will put out it's rating all the time, and won't fluctuate below it.
Next thing is the amps on your rails. You'll want at least 18 amps on the 12v rail. Basically the more amps each rail has, the more power it's gonna put out faster.
And finally, you will want a PSU that will not fluctuate it's rails, you don't want your 12v rail going from 11.5 to 12.5, it needs to stay steady and it needs to stay not very far below 12v.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w