Try using GPUz to get a temp reading of the 4850 it can take upto 100C but it ain't advisable to be there for too long.
So on an average you should be playing at 70C and 40-50 at idle. When the rooms ambient temps are between 25 and 35C.
Freezing can be because of the overclock.
Download GPUz from techpowerup.com , run it, get the screenshots or the log since your game freezes and post it her for a more specific resolve.