I guess that would depend on alot of factors; like what cpu you have, how much ram you have, what type of ram, etc etc. Post your current system specs.
I just upgraded from a 1.8ghz P4 s478 to a 2.6ghz P4 s478 using my same mobo. Windows is a bit snappier and I can go up a resolution (or 2 maybe) in games. Not a dramatic boost. I was scoring about 10600 in 3dmark 2001se before and now get about 13800, about a 30% increase.