Depends on the application I'm running how high it goes without artifacts.
Playing the normal Q3, Half-Life, Unreal etc I can run 150/345 all day without problem. Running any sort or visualization applications and that drops to 140/330 and even then with very large scenes that can start causing problems(135/320 works flawless in this situation).
Oddly enough, the most brutal application I have for artifacts when OCing my vid card is DXball(a Breakout style game). Anything over 130/320 will quickly result in screen corruption.
Pretty much if I was just using my vid card for basic tasks and 3D gaming, I'd leave it at 150/345.