It goes 5858<470<5870<480
If you get a decent case the heat won't be an issue. 805W should be enough for 480 even in SLI. On the other hand, for $400 you can get SLI 460s which appear to be extremely overclockable. $600 gets you CF 5850s which will outperform any single card and overclocks easy. But yeah, for a single card set up the best is GTX480.
EDIT: Can I ask which RAM you're getting? 6gb is unusual for dual channel RAM. Since you say you'll be gaming, you won't need more than 4gb for a long time.