the prices for 7000 series will go down even more come early next year.
so you might want to wait it out until next year before upgrading anything.
personally i think the i3 + 6950 is a serious gaming machine for 1600*900 res.
i know you are experiencing low fps in gta 4 and thinking your gpu is not good enough.
but the truth is gta 4 is a crap pc port that runs horribly across all hardware