It will enhance gaming performance and stuff like that. But it shouldn't make your PC run faster in just regular use. If you feel like 2 gigs is enough, make sure you lower the page file size(you can even turn it off if you have enough RAM, I find 2 gigs is about the size I use for gaming). That will greatly increase performance.
But as I said, more RAM won't give you any speed increase, it just lessens the capacity of the RAM as a bottleneck in memory intensive programs.