Sorry, but I can't help wondering why bother to post a question if you're not going to trust two very clear responses?
Lots of poeple are running 333 Bartons on this board with no problem. The only issue is that the BIOS won't recognise the CPU automatically. Bartons are "supported" on Ultra2s (w/latest BIOS) because they are recognised by the BIOS. On the KT3 Ultra-ARU, you have to set the multiplier manually. On either verion you have to set the FSB manually, so setting the multiplier as well isn't much difference.
Of course, some people have problems even with supported CPUs, so no guarantees!