I ran the Dirt 2 benchmark after upgrading my 4870 512mb to a 5850 only to find my average and minimum fps have increased by 1 (yes that's right, ONE) lousy fps. My processor is an e6400 @ 3.1Ghz and it seemed to be the consensus that it would not be too great a bottleneck (I actually overclocked it more from 2.8 to 3.1 after consulting the forum on my upgrade). I play at a res of 1650*1050 so I thought the memory increase alone would give me a decent fps boost.
£250
1 measly good for nothin' fps

Half of the reason I upgraded was because I'm getting a 24" monitor this week, but still...
£250
1 measly good for nothin' fps

Half of the reason I upgraded was because I'm getting a 24" monitor this week, but still...