An Nforce 2 Ultra will be a revised version of the original NForce board and should have had any issues ironed out. That is definitely true for the Abit NF7-S, which is also an overclocker's dream and very stable. They also have SATA, Firewire, and a parallel to serial adapter. IMO the Barton 2500+ was made for this board... I can't speak for other NForce 2 boards, but any of them would be better, IMO, than a KT600 one.
The KT600 boards are new, and new boards almost always have some problems. Notice that Boonesmi hasn't even gotten his board yet. I can't see why anyone would go for an untried board, and pay more money, just to get better audio (at a CPU hit I bet), which you probably won't be able to tell the difference anyway.