It would be helpful to know what video card you're running, since they tend to have significant power draw.
If you're running a simple system with integrated graphics, you can use a basic 500W like the Corsair CX500. I've used this before and I quite like it.
I would get a little more headroom with the CX600 if you plan on upgrading or adding on in the future.
If you're running SLI or crossfire, get the CX600M or something similar.
If you had integrated graphics you would only need a max of 400 WATT