Actually I found more info on your PC, it already has an Asus M/B in it, here's the specs:

http://h10025.www1.hp.com/ewfrf/wc/d...ame=c00006476#

This motherboard has an AGP slot, so you are barking up the wrong tree looking for a PCI card. Get a good mid-range AGP card, ATI Radeon 9600 or 9800 series or Nvidia GeForce FX 5600/5700 Series should work well.