basically their is no need to overclock
955 can handle 6850 efficiently , it can also handle two 6850.
but their are some games that demands more cpu than gpu, then you should consider overclocking 955 (as it is a BE, made to be overclocked)
use task manager to determine whether you need to overclock 955 or not
Test your rig and games with all at stock settings and see the results, after that, overclock the CPU (3.8GHz+ or above) and run game again to compare the results against the stock ones. Choose the one that gives you better performance.