Repeated Machine Check Exception Errors - Number Codes Change

In the past few months I've added a 512 stick of RAM and replaced the hard drive in my five year old laptop with a brand new Seagate Momentus. The laptop itself is a Pentium M 1.8 running Windows XP Home (Service Pack 3). Also in the past few months I've started receiving BSOD's for Machine Check Exception errors. The numbers tend to vary, though I can never catch one long enough to write them down, as the system flashes the error screen and then shuts off or reboots.

I've run MemTest as recently as this afternoon and the results came back with no errors found in the memory. I ran a SMART test from SpeedFan on the new HD and it was deemed perfectly fit. Still, I can't shake the feeling there's something going on with the new hardware in this laptop, as I had never even heard of a Machine Check Exception since I never got one until some time after installing the parts.

I notice I get this specific error when playing a computer game (Diablo 2, for Christ's sake, so the game isn't particularly taxing!) or watching video, ie intensive activities. It never happens while just browsing the net. I am not overclocking the processor. In fact, I'm undervolting it to keep heat down, though I've turned off Notebook Hardware Control to go back to normal voltage setting and I still get the Machine Check Exception errors with the same frequency.