Re: Weird graphical trash/hard lock. 6600 AGP
I bet you do have a heating problem on the GPU. I have a 6600GT AGP, and I had lots of similar problems and it was all due to GPU overheating.
To monitor the GPU temp (on the 6600GT anyway), you have to edit the video bios to turn on the temp sensor (why the default is disabled I don't know). Look for a windows program called "Nibitor" (it runs fine under vanilla wine if you don't have windows). Dump your BIOS out to a file with the nvflash utility, and use Nibitor to edit the temp sensor bit. Save the edited BIOS, and flash it onto your card. After that, nvidia-settings will display and monitor your GPU temperature.
I eventually solved my problem by installing a bug copper head sink/fan combo in place of the fectory one. It's run great every since.