If this is your motherboard:
Motherboard Specifications, PTGD-LA (Goldfish3) - HP Customer Care (United States - English)
your choices are limited. It has no dedicated graphics card slots, so the best that you could do would be a PCI card. Most cards at this time are PCI-E (PCI-Express), which are entirely incompatible regardless of the similar-sounding name.
What are you using for
drivers now? There are no Windows 7
drivers for your onboard (915) graphics. The newest
drivers are for XP, and don't support Aero Glass.
Here's a list of some PCI cards available at Newegg (if you're in the US or Canada):
Newegg.com - Computer Hardware, Video Cards & Video Devices, Desktop Graphics / Video Cards, PCI, DirectX 10, DirectX 10.1, DirectX 11, Retail
The cards are usually much more expensive than their PCI-E equivalents. Whether it'd be worthwhile to play a game from 2005, only you can say.