Upgrading Mobo

[AllynTal]

I'm helping a friend get some more life out of his old Compaq. I want to get him better performance for gaming. Right now, his frame rate in game is in single digits and frequently goes from frames per second to seconds per frame. His motherboard only has PCI slots. I want to change out the MoBo to something that has PCIexpress slots so I can get him a nice vid card. We don't have an unlimited budget; if we did, we'd just go buy a fancy gaming box. We have about $500USD to spend total for the Mobo, vid card, and chipset (if there is enough left for a chipset).

At present, he has this MoBo:
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00300046&lc=en&cc=cl&dlc=es&product=453536

I want to replace it with something that will accommodate his current chipset with the intention of, in future, getting a fancy chipset. The current PS is, I think, 450W. It isn't less than 400W. He's got 2G of RAM and running Windows XP Home.

I started researching an ASUS P5N-D LGA 775 NVIDIA nForce 750i SLI ATX Intel Motherboard with a GeForce 9800GTX card but all the specs are making my eyes glaze over and I soon realized I need expert advice. I come to y'all with hat in hand looking for recommendations.

 

[AllynTal]

UPDATE:: Nevermind. I understand now that I can't upgrade the motherboard. Since I can't find a way to delete the post, I'll just wish everyone a good day.

 

[fullmetall]

Yea, kinda hard to change things that were built by a manufacturer. You can still build a rig for $500-$650 then upgrade little by little.