Damaging electronics is always a possibility, but I have found that they are surprisingly resilient and usually can resume normal operation when returned to their rated environments and operating limits.
Indeed, most likely your problem was simply due to overheating and by returning it to normal speeds, the artifacts would go away.
I don’t know much about 3D Mark (especially if you have to visually watch for artifacts), but I would recommend ATITool and ATI Tray Tools. Both allow fine-grained control for overclocking, detailed tweaking of countless software settings, temperature monitoring, plus many more features, most importantly—for this question—is their automatic artifact detection and auto-overclocking functions. Plus, both provide direct access to the devs through their forums, allowing you to quickly and easily get support and even influence future versions.
I ran a FurMark test for about 5-10 minutes and didn't see any artifacts. The hottest it got was 89C. Think I'm okay, or do I need to let it run for a few hours or something? – Mike Pateras – 2011-01-09T07:03:48.280
@Mike let it run about another 20 mins, if all's alright you should be fine – Sathyajith Bhat – 2011-01-09T07:24:12.903
Do I have to watch it the whole time? Or can I just come back in 20 and see if any artifacts are appearing then? – Mike Pateras – 2011-01-09T15:12:53.373
@mike best if you watch through the entire 20 minutes – Sathyajith Bhat – 2011-01-10T03:15:30.227