I also had problems with my ATI Radeon 9550 AGP card freezing up my system whenever I tried to play a 3-D game. I have the Dell Optiplex GX260 and my PC would either freeze or I'd get the dreaded BSOD. Finally, the only thing I found that fixed my problem was uninstalling the ATI driver (EVERYTHING, including the Catalyst, driver, and chipset) restarting the system and replacing with the modified Radeon driver from "Omega". (google search omegadrivers.net) It's an updated version of the old ATI Catalyst 7.12 but this one is modified for those of us with the AGP system crash issues. Just be sure to choose the correct version for your Operating System.
Good luck!
Jake