Running in 8X, the AGP signal voltage is 0.8V, not 1.5V.
AGP aperature should be set to approximately 50% of your system RAM, if it goes that high. Since you're running 1Gb of RAM, like me, set it to 256Mb
First things first though... make sure that it's not a driver problem causing the lockups.
Go here and download the Catalyst 3.5 drivers and the Control Panel.
I've found that the Catalyst reference drivers makes for a more stable and better performing system than using the OEM drivers. You won't need to install the WDM capture drivers unless your card has video in.
Install the drivers first, reboot, and then install the Control Panel.