Okay. Start by seeing how far you can go on stock voltages. You should be able to get to 2.8-3.0ghz with these new 6600's. Once you reach your max on stock voltages bring them up by .0125 or more depending how far you want to go. 3.6ghz would net you the best performance per clock sincen your ram would be synced with the fsb but you may have to set the voltages up to 1.5. Thats not horrible, and the cooler will handle the extra heat fine, i'd imagine at those voltages you'd be looking at getting a new cpu in about 3 years.
Others more experience than I can go into more detail, as ocing isnt my expertise.