Something important to remember when dealing with power supplies is, "Quality takes precedence over quantity".
My system uses a Fortron Blue Storm 400 watt power supply..
I have an nForce4 sli board, X2 3800+ (stock), four sticks of memory, an X800GTO overclocked to a mild 500/500 with the remaining four pipes unlocked, a hard drive, an optical drive, two 120mm case fans and an add-in sound card.. 
I have absolutely no issues with this power supply.  Clean, stable power, no blue screens, no freezes, no hiccups
A good 350 - 450 watt power supply is all you should ever need..
I would assume you only need more if you intend to overclock your CPU, in addition to running SLI, and have multiple hard drives and optical drives..