that's a bit of a dinosaur these days
However, there are some newer Radeons in AGP form, and there are newer Athlon 64's than the 3000+ that use the same socket. Depending on where you're from you might have trouble finding older hardware at reasonable prices, since the dealers here go crazy and slap 'Legacy' tags on anything older than a year or two.
You could (probably) find parts to replace most of this PC, it's just a case of:
1. i'm not sure of the actual performance increase
2. i'm not sure if it would really be worth the money you'd be spending