That's a very out dated machine. The best processor you could get would be either a AMD Athlon 64X 6000+ (3.1Ghz) or a AMD Athlon 64 FX-62 (2.8Ghz). But its not going to be much of a upgrade. These CPUs could not handle any graphics card good enough to play modern games. The only thing you could do with this machine would be slow web browsing or minecraft.
You could build your own computer for $400. It would be quite an upgrade. Just keep that in mind.