Well, I hope you enjoy your PC for more than displaying cool screensavers :). You'll need see if you can disable your on board video in the BIOS and then find a card that supports openGL. It's also likely that you have no AGP slot since your Intel on board device will be using this channel and you will need to find a good PCI version. These are available but not always readily available. You'll need to open your PC to see what available slots you have. Check for the ATI and Nvidia offerings for ones that will fit your machine.
Just got my brand new dell dim 4700 w/ XP..loaded. OK, so now I copy my screen savers and find my OS dosn't run openGL. I have an intergated Intel graphics media accelerator 900 v. card. Does anyone know how to upgarde my card to allow me to view my Screensavers(and other 3d apps I guess)..TIA