The AMD XP+ CPu's I believe ran with only AGP cards. If its onboard, its likely to be either an Intel or ATI chipset. Its fairly straightforward to turn it off in the BIOS.

If I had to recommend a graphics card, I'd get either of these.

ATI Radeon X800XT
Nvidia Geforce 7600 (7800 is too much money)

Both are 256MB cards with decent speed GPU's. Where AGP uses some system memory, it would be better to get as much as you can afford as I said above, I personally would splash out and get 2GB and the ATI card. Where they are now owned by AMD, you may be in a better position with regards to driver updates and software add ons.

The crucial.com system scanner is here - http://www.crucial.com/systemscanner/

Belarc Advisor is a utility that can tell you all your current setup. Download it here - www.belarc.com - this will tell you whether or not you have a PCI Express slot (doubt it myself) and how many free RAM slots you have just to confirm what hopefully the above scanner will tell you.

Cheers

Matt