ATI Radeon 7500 is a great PCI card, and not too expensive. I had one in my old (just got rid of it a week ago) P3 833, and I could run all the newest games (Halo) on the lowest settings just fine. Halo was rough at times, but playable.
If you want to play games and dont care what settings, just save your money, keep your mobo and get the card i mentioned.