If any of you want to provide a little "try this" tech support:
The card: AGP ATI Radeon 9100 (essentially a 128MB version of the 8500) that I got dirt cheap.
The OS: WinXP SP1
The BIOS: Award, currently with 4x AGP enabled, and an AGP apeture of 128MB
Monitor: CTX4100 with the latest drivers.
The exact problem: It boots fine after installing the card, but when I install the drivers and reboot, it hangs when logging on. After the first few reboots, it just goes black after I select my account from the logon screen.