Maybe the BIOS makes its memory timing decision based upon the new stick? You could try swapping the modules between sockets.

Just recently, I've been fixing numerous systems with a patch that addresses problems introduced by KB916089. The patch appears to be pretty safe and it performs a pre-installation check to determine whether it should be installed. I've 'fixed' about 8 crashing PCs in the past week with this magic bullet - definitely worth trying IMO.