You can usually ballpark the number depending on your setup, just make sure you get a better power supply then the one that comes with your case. Thermatake and Fortron I hear are good companies. But like I said, it depends on your setup, most non-SLI setups usually get a 400-450W and run with no problems and plenty of room for future parts without having to worry about PSU problems.
EVGA 680i SLi
E6400 @ 3.0 GHz
4 GB OCZ Platinum Rev. 2 EL @ 880
400GB + 250GB Seagate Barracuda SATA2
Antec P180B Case