Most roms are 800mhz OC because almost all Droid's can handle that. Seems like a fair number can handle 1ghz.. and a decent amount 1.2+ghz. UD is 800mhz, kernels allow you to get up to 1.2ghz. It also allows you to do low and ultra low voltages, meaning that the phone will end up using less battery and have less heat. Also SetCPU is just the governor (I think) for the CPU, so you can send profiles, and make sure the CPU isn't running at the OC speed at all times.