Help me OC my 4850

[Rydawg5143]

So i finally decided im goign to OC my system ( in sig) and figured i'd start with the gpu since it seemed the easiest. So i get into ATI CCC Overdriver and clock it up, run furmark stability and my screen goes blank with anything above 650/1045. The computer syatys on but screens say no imput and i have to restart comp to work. I made several profiles in CCC for 30% 50% and 65% fan speed ( anything above is to much)

How do i get a high OC w/o crashes? up voltage? how? ima noob to OCing