OK, your comp has noAGP slot for a graphics card, so you'll have to go with old school PCI.
The best you're gonna get is a Geforce FX5500 256MB 128-bit DDR PCI Video Card. NewEgg has one from 3D Fuzion for $83 + $2.50 shipping.
If you are wanting to game, you're gonna need more RAM as well. NewEgg has a stick of 512MB from Patriot for $37.55 + $4.81 shipping.
So, for $127.86 you can turn your old computer into a decent low budget gaming system. Ih, it won't be a screamer by no means, but I think you'll get plenty of enjoyment out of it.
Not everyone has to have bleeding edge stuff to be happy with it.