If you have the core 2 duo, then keep it. Like esrever said, games are just now starting to use more than 2 threads. If you can OC, that is the way to go. If you are playing games like BF3, Mafia 2, Crysis 2, or the other few that use 4 threads, go ahead and get a Q6600 and OC the heck out of it, but I wouldn't really suggest upgrading the LGA775 platform, as it is pretty old.
If you can, OC your duo and save for a IB or SB platform.