There's a reference/recommended list here: http://www.silentpcreview.com/article699-page1.html
Most people will agree that anything above a 7 in quality and noise on that list is fairly good.
Two things I see:
1) You don't need 600W for that system you're proposing. Even a 300W PSU can handle what you're asking. Anything in the 350-450W range on the aforementioned recommended list will be fine unless you're going for SLI. However, SLI isn't exactly the most economical way of upgrading. It's generally better to stick with one graphics card. I'd recommend a 380W Seasonic S12 or 380W Antec NeoHE, which is a little cheaper than the Seasonic.
2) There's no such thing a 'silent'. Well, there is, but I don't think you want that, especially the definition of 'silent' in these forums. What I think you want is a quiet
PSU (feel free to correct me if I'm wrong). Be careful how you use the word 'silent' around here. If you want a real silent PSU, you'd have to look into passive (no fan) PSUs like the Fortron Zen or an external brick PSU.