This article has some good information comparing the GTX460 to various other cards.http://www.guru3d.com/article/geforce-gtx-460-review/1
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_11_support
Your 850 watt PSU is more than enough to SLI a pair of GTX460's.