Make sure you really test it hard though.
My 4850 looks fine at 700MHz. It'll run Artifact Finder for hours without issue. It'll even run through Crysis at Very High without any problems for at least an hour...
...but if I set the same settings as HardOCP recommends for gameplay, it'll crash the driver (and because Vista is awesome, NOT crash the computer) after about 10 minutes of gameplay at any frequency over 660MHz.
I suspect the voltage for the core is not sufficient because my third party cooler is keeping the temps down.
Exactly. Open up FurMark and set it to: fullscreen, stability, 1920x1200. See how long things last.