Run prime for about an hour and see what the maximum temp is in real temp afterward. If it stays under 60c's then you have some headroom for a nice overclock, at least 3.6:exclamation::money::super_happy:
It's safe to stay under 70c's load, but there are people who run 78c's load 24/7 and don't damage the q6600. It is a monster and can take tons of abuse.
Also, yes speed step is probably the cause. Just turn it off imo, although I think veedub left his on when he had his q6600.