4 gigs of ram is a bit much. i probably wouldnt go further than 2. past that, you probably wont notice a performance increase, just a bigger hole in your pocket.
a compy with a newer bus and proc would be a diff story tho.
i would check your hard drive and see how large the cache is. this is often over looked and makes a huge performance increase, especially during heavy disk thrashing. if its the hdd that came with the computer, i bet it has a 2mb cache. 8mb would be my recommendation, altho they do make drives with larger caches that that.