CephaloP
You need a PCI card... OR.... you coould buy a new mother board, one with an AGP slot. Just make sure you have a friend come over to help you. It is VERY easy to replace a mother board, but then, its also very easy to drive a sports car over 140 MPH! LOL
Seriously though, motherboards can be upgraded for decent prices.
BUT! Be careful when/if you buy. Make sure it will fit your case. Manufacturers sometimes make their "box" or "case" proprietery. Made in such a way that only THEIR mobo (motherboards) will fit. I ran in to that problem once, and then just threw the case away, and bought a new one.
You can find cases ranging in prices $25 ~ $250 (or MORE!) I made mine using a $29 case from Fry's Electronics. Its small, but most everything fits (LOL I did say most, my DVD drive sticks out about 2 inches from the front of the case because the case I bought isn't an ATHLON case, and my system RAM is in the way. So I have this handy little shelf to sit my sunglasses
).