the only reason I care a lot about my CPU and not much about my GPU (running a MSI 480GTX) is that DayZ and Arma games are very CPU dependent. A lot of MMOs can be as well. But, if you have a top tier i5 I wouldn't see much point in going up. maybe just more RAM.

For me I have the best i3 that was out at the time so if I'm going to upgrade I'm gonna go for an i7.