What exactly are you performing poorly on that you need more?
You are correct more RAM would not really help.
You could look into tweaks (
http://www.tweakxp.com/) or you could look into Overclocking.. depends on how comfortable you are and what exactly you expect.
Do you want better gaming?
Do you want better benchmarks?
If you want better gaming, what vid card do you have?
btw, there isn't muchyou can do with a system like that where you'll get a noticeable increase.
Also look into cutting back on Windows services.. there are a lot that can be stopped and are nothing but resource hogs.