Since the phenom ii 720 cpu is a black edition cpu belonging to the same die of phenom ii x4 you can try to unlock the fourth core if it is not bad.The new 890gx chipsets along with easy core unlocker functionality are the best for your cpu.
If your budget is less,go for the 785gx and 790gx/fx motherboards with Advanced clock calibration and resp. southbridges.
Search toms hardware for the complete set of motherboards which can successfully unlock your triple core cpu to quad core.Still chances of unlocking may depend upon the any motherboard updates and the health of the 4th core.Still you can give it a try to unlock.