In one of my PC's i have a X800XT PE OC'd from stock 520/560 to 580/600. I have it cooled with an artic-cooling silencer 4. When I installed it i didn't use the artic cooling thermal compound, instead i used the ole faithful, "AS5". I've got it on the GPU and all 8 RAMS as instructed with cooler manual. Even at these speeds the highest i've ever seen the GPU was 54-58C.
I've been running this card at 580/600 for the last 2 weeks and haven't had any problems yet. No artifacts, No Freezing, No reboots, No Crashes, nothing... Hopefully i didn't just curse myself. Knock on wood.
When it comes to OCin' you much take proper cooling precautions.
Or, don't bother, because you will have problems. Period.