Not sure about how much juice the gf4s eat up, but a good strong 350w would probably be ok.
I'd say get a quality 400w (Antec Tru-power, Enermax, etc), but if you feel daring....I use compusa 300s in a bunch of rigs, and they are pretty solid.
But if it is to be your main rig, why cut a corner? I think that a good mobo is the most important thing, and a good psu is next.
With good performance out of those two parts, you can make a bad chip look good. A bad psu will make a great stepping a lousy overclocker.
There is a thread about psus around here....but If I remember right, the +5v and 3.3v lines (?) should have a combined total rating that is close to the capacity of the psu.
Somebody correct me if I'm wrong here, please!