More RAM sometimes helps depending on which OS, what software you run and how much memory you currently have.
Faster hard drive maybe. If yours is a 5400 RPM you can replace it with one that runs at 7200 RPM.
Processors are bit more difficult to get at. You didn't mention what your current processor was so it would be difficult to determine what performance increase you might get with any of these. There isn't that much clock speed difference between the 2.0GHz and 2.4 GHz to, IMO, justify 175 bucks more in price. As well, it won't be the bottleneck if you're utilizing the HD's swap file regularly.
Does anyone know how much performance increase would be seen by buying a laptop with these different processors? Where is the upgrade not worth it? Thanks for help in advance.
Intel

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic