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.