oh, i forgot to ask in my previous post, why do you want a mobo with support for 140w tdp cpus?
your x3 720 has a tdp of 95w and the only other current amd cpu (i think... i'm almost completely sure) that has a tdp of 140w is the c2 stepping x4 965, but amd has already released a c3 stepping x4 965 with a tdp of 125w which costs almost the same.
also, even if you were to use two 5870s in crossfire, there would not be a huge difference in performance from running cards in crossfire at x16/x16 vs x8/x8.
ATI HD 5870 Scaling Performance: X58 vs P55 Showdown - AnandTech :: Your Source for Hardware Analysis and News
but, if you still want a mobo with those req's, this is probably the cheapest one you'll find (on newegg anyway)
Newegg.com - DFI LANParty DK 790FXB-M3H5 AM3 AMD 790FX ATX AMD Motherboard - AMD Motherboards