I recently did this, but haven't run any benchmarks.
I'm doing it half as an upgrade and half to build a new computer and use my existing rig (in sig) to give to the better half.
Basically, I went from running the E2180 @ 3.4Ghz for benches, or 3Ghz day-to-day to an E5200 running @ 3.8Ghz (limit) or 3.5Ghz day-to-day.
Obviously, I've not noticed any change but all I really do is internet + WoW at the moment.
As a gamer, I'm seriously doubting anything over a 3Ghz dual core is needed if you only run one graphics card, and by that I mean it won't give any framerate increase.
You could go for an E7300 if you want a better shot at getting to 4Ghz, but I couldn't justify it for the price premium.