I have an AGP nvidia 5900 kicking around somewhere, that kicked ass back in the day.
Iexion, sadly, it is not worth spending even one cent on that machine. You will struggle to find anything compatible in the way of upgrades, and what does exist will be almost as old as what you have. In order to use a modern gfx card, you will need to upgrade the motherboard, which means you will need to upgrade the memory and processor too. Your case probably doesn't suit new motherboards either, so a case and power supply will be needed on top of that.
Consider getting a complete machine. I've seen some pretty awesome deals on the ebay for around $300. Powerful machines for hte money.