The fastest CPU for single and dual threaded applications would be the E8400, and would overclock well too. The best for multitasking would be the Q6600, and would be a very nice quad core if you could overclock it to 2.8 GHz or more. If you want this rig to last you a few more years then a Q6600 would be the best choice. I'm not very good with socket 775 boards though sorry.
The Maximus III Formula really isn't worth it imo. You're better off with the ASUS P7P55D-E Pro.