Originally posted by: Coherence
If it's only during Halo, check for a patch. If that doesn't work, then I'd check for a video driver update. Next would be to check your CPU and GPU temps, as a 3d-intensive game may be overworking your system and require better cooling (I recently replaced my video card cooler; they don't last forever, after all, and my card isn't even as old as yours).
I wouldn't expect bad RAM to cause a spontaneous reboot. Bad RAM is more likely to cause a STOP error of some kind without a reboot, in my personal experience. But, it can't hurt to check that, either. RAM testing software is a pain in the arse, IMO, though. I've yet to see one that runs reliably in Windows. And since my rig is floppy-less...