I would like to detract with the other posters on one issue: Yes the memory upgrade will make a difference, especially if you are gaming. Going from 512MB to 768 or 1GB does make a visible difference.
Is gaming what you are doing with this system? Also what kind of video do you have? I would not just dump the mobo/core like other recommended. This system still has life in it, and you can always transfer a next gen video card and you memory to a newer system if you want to do that lower upgrade process. With AMD's chip integrated memory controller, people are finding using even older PC2100 RAM on a new Athlon 64 still results in tremendous performance increase. So keep your system for now, upgrade RAM/Video etc. and then transfer parts to old computer when you make the move to A64.