Ignore all of the above.
Your cooling is simnply not good enough. Firstly check the back fo the card. If it burnsyou, or infect hjust if it feels hot, then its too hot. Arctic cooler will be a good idea (MUCH better than stock cooler - I cant belive youd overclock an ATI card with those CRAPPY stock coolers. A pierce of turd cools better (runny turd anyway, water has a high specific heat!!!!!)
Stock voltage for your card is ~1.7v which is ok for 400-450 on a lot of cards, 9700's ually manage, but 9800's seem to need slightly more. If your cooling is good enough you can vmod it. If your not skilled enough to do vmods then
imho you hsouldnt be o/c atall!