20-25A on the 12V rail from a quality PSU would do. Most 350-400w Quality PSU's would do for that setup. If you upgrade the GPU you would need something with more power, so getting a 400w+ PSU would allow you to keep the PSU longer and not have to spend more $ next time you upgrade.
350-400w Quality PSU now with current setup (20-25A on 12V rail(s))
400w+ for future upgrades (Antec 430w EW/Corsair 450w/PCP&P 470w)
A Corsair VX-450 would be a good choice, if you want a little upgrade room a Corsair VX-550 or HX-520 are good choices. You could also look at the Antec Earthwatts or NeoPower 500W versions. The Silverstone ST50-F is a good 500W choice too, or the PC Power and Cooling Silencer 470W.