I successfully unlocked the 4th core on my X3 720 and overclocked the CPU to 3.6ghz. Voltage stays the same with the 4th core unlocked, at 1.41v, but I noticed that the CPU temp went up about 4c from 50c to 54c. I guess this makes sense because now I have four heat producing cores instead of just three.
Note that the CPU temp is the one from the motherboard, not the core temp. With 4th core unlocked, I can no longer monitor the core temp, which tends to be about 5c lower than the CPU temp reported by the mobo. So I estimate the core temp to be 49c unlocked, versus 44c with 3-cores.
I ran some benchmark and used the computer with 4-cores for a few days and it seems that having an extra core doesn't do anything. There is no noticeable speedup whether in office tasks or in video games. So is it still worth it to keep the 4th core enabled despite the higher CPU temp?
Unlocking cores disables the Core temperature sensor, which is why you can no longer see it. If your primary use is Office Tasks and games (which except for a few exceptions show no real benefit from 3 vs 4 cores) then it would probably be best to go back to three cores. It will run cooler (so last longer) and use less power (though if you are using it as a space heater, then that is a bad thing).
Few tasks are truely multithreaded yet, and only use 1 or 2 threads. Than most games that show benefit going from 2-3 cores, shows almost no gain from 3-4 cores. You'll only really see a difference in a number of applications that are 4+ threaded.
It's probably best to just keep it as a tri-core for now and maybe two-three years down the line (if you're still using that system), it would be worth unlocking the fourth core. Hopefully applications will be more optimized to make use of the 6-12 cores CPUs will have in the next several years.