If you're doing CPU-intensive jobs like 3D rendering, compile jobs, Photoshop, DivX ripping, etc., then yes, you'll probably notice a difference. If you're just playing games, using MS Office, surfing the web, and checking e-mail, then you probably won't notice a difference without running benchmarks.
Kelledin
[dave@discovery ~] kill -9 1
init: Just what do you think you're doing, Dave?