Doubt over clock killed yours, iv been running my 6850 massively over clocked for awhile now with out a problem, Link in the sig to gpu-z. Only way you can damage it over clocking is, Over heating it, Over volting it too much or running it hard for hours on end even tho its artifacting/glitching and finally Electromigration which takes few years.
If you have to have CCC enabled you can still use clocktweak to change the cards individually.