250W will be fine (actuallly 200W will do).
I'm running an Athlon II 635 (2,9 GHz QC), 3 GB RAM, 250 GB, 500 GB & 1000 GB HDDs 7k RPMs, BD R/W, GeForce GTS 450 (which I'm running full load with F@H) with - 340W. Would've tried a 300W but couldn't find a decent one.
Just add up your loads, add 10% for safety (or 20%) and that's it. On heavier 5V loads you'll need to watch that rail specifically. And you'll want to get a quality PSU before you risk a cheap one burning your precious hardware to a crisp.
And with the probably rather fair load an 80+ PSU will save some energy when running long hours.
