Why not consider the Q6600 for the 4th core? Only $50 more than the 720? And on the above benchmarks, out preforms the 720 on anything multi-threaded. And unless you are going backwards in tech the number of programs using more cores is only going to increase... (i only ask because it is my delima too!)
It is a notoriously high overclocker, but only drawback is 65nm, but I assume you have decent cooling to counter any adverse effects...
What mobo would you OC with?