Seasonic S12 380 is definitely quieter than the Antec SmartPower 2.0. It is also be able to provide enough juice for most PCs. It is obviously not a SLI certified PSU, and install something like a X1900XTX should not even be attempted.
If you want a quiet case and PSU then check out the Antec P150. It comes with a 430w NeoHE PSU that is almost as quiet as the Seasonic. It's selling for under between $110 - $120 before shipping.
Older NeoHE PSUs has had problems with Asus motherboards in general, but was fixed when newer versions were shipped back in February.