I currently have a new ATi XFX 4890 and have been doing some overclocking with it. Sadly however if the clock goes above 910Mhz when loading up a game or 3D Mark Vantage the system hangs and I have to restart. I would normally expect this to be with it getting too hot, but when leaving it on with Furmark it only reaches 70 at max with the card clocked to 970. My PSU is 650 Watts and The card uses stock cooling.
I would try a more modest overclock. The 4890 is a great card even before being overclocked. Some programs/games are more stressfull for a video card than others. If you really want to overclock then I would say buy a third party cooling fan/heat sync. OR for a more extreme overclock I'd go with a liquid cooling system.
Also make sure there is plenty of air flow in your system.
Typically with an overclock it's a heat issue.
Do you have any issues in games or 3D Mark Vantage when you are running default clock settings?
If you answer no, then it's probably a heat issue.
If you answer yes, then I would make sure your power supply is providing enough amps on the 12 volt rail.
No it's fine normally I just expected it to be able to go higher with people claiming speeds of 1.2Ghz and above. Anyway I found the reason, it needs a voltage tweak but I've got the standard XFX version with no VRMs (I think that's right) and I can't change them, oh well it's still an awesome cards, it was just that some people said that you could get the same performance of a GTX 280, and that it was fine in Furmark.