there should be a setting in your bios, *cpu/pcie sync/Async*,
its in the cpu setting page on mine, change it to Async and it will not be Overclocked when you affect the cpu,
Re:And what frequency is safe for oc ??
That depends very much on your chip,
Each chip is different, one 965BE will O/c to 4.2GHz and be fine, the next will struggle to get to 3.8GHz,
Just go slow, read lots of guides on here and elsewhere on the net, and change tiny things at a time,really tiny,
if your not sure of something, read up on it BEFORE you change it ok
Moto