I took the liberty of entering your specs (guessed at a few things, HDs etc) and came up with this: 296watt recommendation. So I'd say at least a 400w will work for you machine. Granted you can be more precise with filling out your system on that site.
Also it's a good idea to look for a few things when purchasing a PSU.
1. Active PFC (Power Factor Correction), it's hard to find PSUs without this now-a-days but your should still be aware of it.
2. 12v Rail. having more reduced volatility since the power drawl is spread out over more rails, having 2 is usually recommended.
3. 80+ cert. I wouldn't by anything that doesn't have a bronze or higher 80+ certification. You can see the details of the bronze - silver - gold ratings here: http://www.pcgameshardware.com/aid,692532/80-Plus-What-...