May I ask your reasons for upgrading? Just to get some more power ? Dx11 ? specific issues ?
4850 is starting to seriously chug in some games now and my monitor's native res is 1920x1080 so I just want to keep playing at that level. TBH I think it's more the 512MB memory that's the problem because it really can't handle more than 2AA in most newer releases and provide a playable frame rate at that res.
I'm not sure about all the features that are supported. I know ATI seems to have better DX11 tesselation support and Nvidia has PhysX but in all honesty do those things really matter?
I do think the card is bandwidth starved at that resolution, but I think it's more the card showing it's age due to the architecture.
I would buy the 6850, as of now PhysX support is being leapfrogged by DX11 support. I think the last showcase PhysX game was Mafia 2, and except for the new batman game, which might
have PhysX, there has not been much else to suggest that it will be prominent this year. Not to mention you'll need two 460's to get above 30fps with heavy PhysX, in the case of Mafia 2.
DX11 has already been confirmed for two games this year, DA2 and Shogun: Total war. Add to that many more games that already use DX11, I would get the 6850 since it has better DX11 performance. Looking at the long term, over clocking the 6850 will most likely yield better results then an over-clocked 460.
It comes down to better support really. Both have 2 year warranties, but the zotac can extend up to 5 years. Since the cards are so close in price, that is what it comes down to.