If it only happens in X3, it may be a driver issue, had something similar going on with Toca Race Driver and Bioshock on an older system. Fixed by installing a different driver, older or newer.

If it happens with games in genera, again maybe a driver, or it may be something wrong with the video card. Check that the power connectors are well in place, and that the temeperature does not go out of bounds. Ati Catalyst Control Center has a function that automatically shuts down the GPU if it goes over 90 degrees.

If it happens outside of games there may be a lot of things causing it.

Also, it should be noted that the issue has only popped up recently. It never occured when i was using the integrated graphics in the cpu.

If you were using the integrated graphics before, it may have a conflict with the video card. I'm not that familiar with solutions for that sort of problem.

On a long shot, have you installed any strange DirectX versions or unusual video/audio codecs recently? They may also cause problems.

Onboard graphics are already disabled. Made sure of that when i installed the card.

I have not installed any weird codecs or Direct X versions. Just the most recent DX 11 release when i got the video card.

At this point it is just a waiting game. I wont be sure if my problem is fixed for a couple of weeks.

Sometimes rebooting is caused by the  cpu overheating... maybe the fan on it died..  you might have accidently unplugged the power to it when installing the card.

CPU fan is still chugging along. I think that the amount of ram shown in memtest is significant because the crashes in X3 have only started since the latest patch which enabled the use of more then 2 gb of ram.

I'd roll back to the onboard graphics and see what happens...

Perhaps a power supply fault or... certainly worth checking... a CPU overheat? Perhaps the CPU was on the verge of over-heating, the dedicated GPU increases the case temp and *boom*, your cpu now overheats.