Either the HD 7970 or GTX 670. If you want to stay on the cheaper side, HD 7850 2GB or GTX 660 would still be a significant upgrade.
If you order the 7970 now, you get 3 games with it too. HD 7850 has two games.
Uhhh, going from 285? GTX 680 would be overkill, and you're CPU will bottleneck it...
In CPU limited games, yes. In GPU limited games, no.
I have a firstname.lastname@example.org and I could still use more GPU power than the 6950(7850 equivalent).