Here's a handy dandy PSU wattage calculator:
http://www.jscustompcs.com/power_supply/
It doesn't have as many options as I would like, but it works for the most part.
I've found that purchasing a good PSU with a high wattage benefits you in many ways in the future. Each member has a preference for PSUs, my preference is Enermax.
I'm powering 5 Hard drives, CD-ROM, Barton 2500+ OCed to 2.37 GHz, 1 GB Corsair XMS PC3200, GeForce 6800GT, and a NF7-S mobo. With my 410 watt Enermax PSU. I'm sure other posters can put in a good word for other excellent brands as well.