My friend has a GTX 280, hes in the same boat as you. I would NOT upgrade to a GTX 460 (unless you are thinking about SLI), A GTX 470 isn't even a big enough upgrade, to be worth it, in my opinion. You might want to wait till the other high-end fermis are out.
To me, it looks like on DirectX 9 & 10 games and 3D Mark Vantage, there would be a slight gain, maybe just a few FPS. This probably wouldn't be enough to justify the ~$250 price tag.
It also depends on which games you play. If you play DirectX 11 games, then an upgrade to the GTX460 might be a good choice, if not, I would say no. This is a list of games that support DirectX 11 (surprisingly, only 6 currently do): http://en.wikipedia.org/wiki/List_of_games_with_DirectX...
Your 850 watt PSU is more than enough to SLI a pair of GTX460's.
Yeah, I've definitely decided against buying one, and I really don't feel like spending the money on two right now. Everything you guys said confirmed what I thought. Also I figured my PSU was plenty, but that article on here earlier claimed they used 470w at peak, so I thought I'd ask. Thanks for the help everyone.