Yes, a high video card
OC can cause a machine to reboot, and no, artifacts dont only happen when it overheats, they happen when you
OC too far, or have bad memory.
I think something is up with your memory. Have a browse on futuremark's ORB and you will see that the world record holders for 6600GTs with voltmodding and phase change are topping out at 1300Mhz-1350Mhz. The highest I have ever seen was 1.41Ghz with a massive overvolt on it.
Now, what are you using to overclock this? There is a chance that it isnt working correctly.
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Notebook: Dell XPS M1330 Running Fedora 8 Linux (Werewolf)
Desktop: Gigabyte EX38-DS4
Intel Core 2 Quad Q9450 (Yorkfield) @ 3.46Ghz
4GB DDR2-800 PNY
Albatron 8800GT 512MB
Corsair HX-620 PSU
Running Fedora 8 Linux (Werewolf)
Dual Dell 2407WFP 
Drivers, Games, Demos, Mods and Overclocking Tools At AOAFiles