It appears you don't have onboard video but rather a standard agp card, at least what I can tell from hp.
I would suggest a gforce2/gforce2 mx400 or a gforce3 TI200
With the gforce 3 out the 2's are a little less expensive, the gforce3 ti200 is the lower version of the gforce 3, nice card but it will cost around $200 unless you can find a sale, we picked one up for $99.
Your new card will have installation instructions.
Basically change your display over to std vga, boot back into windows, remove the card from device manager, remove any third party video programs that may be installed. Shutdown, remove the current card, insert the new card, reboot, windows should find new hard ware and assist with installing new drivers.