I would say wait a few weeks for more of the 7XXX series to come out (or the 6XX series nvidia).
Stick with what you have until ready to upgrade to the next generation of cards.
There is always a chance to wait and even after the release of the 7xxx or GTX 6xx, but if you can wait for it good. But if you can't, go for a 6950 and crossfire it later.
Or what you can do is, crossfire your 6850, then buy a 7xxx series card after.
no need to upgrade now with that card it will work well for 1 or 1.5 years more without any problems in most games and after 1.5 years i think the hd 8xxxx series will also launch with new tech so upgrading on that time will save your money and that will be more good instead of buying an hd 7xxxx series card:)
agree with the others. I upgraded from a 6870 to a gtx 580, after selling the 6870 it cost me $200 in total for the upgrade (i have same cpu as you too). while it made a difference, I was happy enough with the 6870 and could have used that money for other things. If you can crossfire with your mobo I'd say go for another 6850, or maybe first try OC'ing your gpu.
Thanks for the replies everyone, reckon I'll wait until there's a good reason to upgrade before I go spending money.
I'll look into crossfire, but I think I'd rather something with 2GB vram if I were gonna upgrade.