You'll need a PSU with at least 32A off the 12V rail(s). I input your system into the extremevision PSU calculator and upped the power by about 30 watts because the A8N32-SLI takes a lot of power.
See sig if you're still confused.
The Enermax Liberty EL500AWT will handle your system with 2 7800GTs, but won't do anything more than that. If you're looking for more power, the EL600AWT isn't worth it because you only get a total of 4 extra amps off the 12V rails compared to the 500W version. Pretty quiet. Good efficiency, around 80%.
The Seasonic S12-500 can also handle your system with 2 7800GTs (produces 33A on 12V), and the S12-600 also only gives a total of 3 more amps off the 12V rails compared to the 500W version. Pretty quiet... probably has the edge over the Liberty. Good efficiency, around 80%.
The OCZ Modstreams don't have enough power. The Powerstream 520W and 600W PSUs do, but I don't know how quiet they are. Efficiency is low, hovering around 70%.
The Fortron FX600-GLN supplies a ridiculous amount of current (up to 45A or so, I'd say - their specs aren't very clear) and is supposedly very quiet - no idea how it compares to the S12 series. It's relatively inexpensive too - $125 from ewiz. Good efficiency, probably slightly higher than 80%, claimed efficiency is 85%.