Good lord! So you're loaded, right?
In my experience, there is not a ton of difference in the performance of different motherboards. That particular processor multiplier unlocked, which means it's pretty much made for overclocking, so I'd suggest going with a motherboard that supports overclocking.
http://www.neoseeker.com/Articles/Hardware/Reviews/phenom_ii_x3_720be/15.html has some tips on overclocking that processor from 2.8 to 3.7 Ghz--which is pretty sick, really.
This part of the review might be most helpful:
"I think the AMD Phenom II X3 720 Black Edition is a great chip - and it is especially good for people who like to tinker and overclock, as with a cheap 780G based motherboard and DDR2 you can put together a nice fast overclocked system..."
