Why the artifacts (OC'ing Sapphire HD 4890)

I have a Sapphire HD4890 (stock air cooler) in an Antec 1200 with excellent air flow. I have been OC'ing it and this is the result I have reached.

GPU Core Clock: 1000 Mhz (stock @ 850Mhz)
GPU Memory Clock: 1130 Mhz (Stock @ 975Mhz)
Idle Temp: 43 deg cel.

I ran a 15 minute Furmark GPU Stress test (full screen, 1280x1024, 4x MSAA) and it ran without any artifacts with the max temperature touching 71 deg cel.



The GPU Clock is as high as it goes (CCC does not allow it to go further than 1000Mhz).

Since my load temperatures are very moderate @ 71 deg cel (correct me if I'm wrong), I tried OC'ing the memory clock further to 1150 Mhz, but as soon as I start the Furmark Stress test, I get artifacts (even before temp crosses 60 deg cel.)

1) What is the reason for the artifacts? Is it because the graphics card isn't getting enough power?
2) Should I over-volt the card and push the memory clock further. The max. in CCC is 1200 Mhz, so I have another 70Mhz or so.
3) Is it worth it? Will it make a difference in performance?

Well, I should run the furmark test again and keep an eye on the RAM chip temps this time. I definitely don't want them to be running anywhere near their threshold temperature. If I reduce the memory clock by 10-20Mhz, it really won't make cause any noticeable performance difference, so I'll look into that.

As for over volting and increasing the mem clock, yeah...its not worth it. I ran a 3D Mark vantage benchmark and mem clocks hardly cause any difference.

Just out of curiosity, even though the max for the GPU core clock in CCC is 1000Mhz, is it possible to push it further using a third part s/w or something?