The performance is fine Im just one of those people who like to upgrade when I can
I have a FX-4100 @ 4GHZ
and the hd 6870
Personally I feel you getting a 7850 or 7870 would be a waste of money. Maybe invest in a new higher res monitor?
Though if you MUST get a new GPU, 7850 would get my vote since it's the cheaper of the two...save you some money.