Open GL performance on the KT400 boards is "broken".... under Direct 3D applications, with a Radeon 9700, the NF2 and kt400 are neck and neck, with the NF2 having a slight performance advantage. Under OpenGL, the NF2 pulls away 10-15% above the performance of the kt400, which is AMAZING. Plus, the nf2 is almost guarenteed to be Barton compatable when they come out in february or late january, whereas the kt400 more then likely won't be as that is probably one of the main issues that the kt400a will "fix".
Now, on the other hand, the Asus KT400 can unlock the throughbreds by themselves, which I *believe* the nf2 can't. So for overclocking you might be better off with the kt400.
Also, the nf2 has the ability to increase the memory speed of your system with 2 sticks of ddr installed. This is coming in KT400sa, but it doesn't currently have it. In some memory intensive benchmarks, this increased bandwith might be usefull, esspecially with a 333mhz fsb athlon.