It takes a while sometimes to find your max core and mem. And overclocking your GFX card wont really hurt it, but it will heat it up which could potentially damage it, but slim chances. Heres a screen shot of what an artifact will/could look like. This is kind of extreme.
Listen to me. Don't overclock unless you understand
1) What overclocking does
2) The benefits and costs of overclocking
3) The symptoms of a component that is overclocked too much
4) What type of performance increase you will see when you overclock
5) How to properly overclock
Reading about how to overclock is just as important as experiencing overclocking something. You should learn from other peoples mistakes, not become a mistake by blindly taking a stab at overclocking. Just google for "overclocking" there is plenty of information on the internet.
I got some information from here. The maker of my 9800 PRO also has a FAQ for overclocking in their forums. I read them over. I know the risks. I know what I have to do. I am fine now.