The problems with just upgrading your vid card are
1. Your motherboard may not accept anything significantly better
2. The cpu has to be able to keep up with the vid card and a 500mhz cpu will not be able to handle anything more than what you have.
So, I will agree with most here by saying that you'd be better off saving for a new system. A new cpu will probably not work too well as, if you have owned the system for a more than 3 yrs (back when a 500 was pretty good!) then the motherboard itself will not be able to take anything much faster, plus the vid card issue mentioned above.
It sucks, but when you're getting to the point where a mb upgrade is the best bet, it's pretty much a whole new system you need. 1.4+ghz CPU, GeForce4 TI 4200 / Radeon 9000 PRO + vid card, DDR ram, modern hdd, etc etc.
It's happened to me more than once.
I love discussing this stuff. If you want more detailed advice, by all means shoot me a private message.