1) Tyan is really the only current dual amd mobo maker (for the MP's) and their boards are pretty good, so i would recommend them as well
2) AMD MP's are the only amd cpu's approved for SMP, but XP's will apparently work in dual configurations as well
3) remember, in addition to needing an OS that supports multiple processors, you'll also need to use applications which support SMP. the biggest examples of that are programs such as photoshop, and some games such as quake3. for "normal" usage, you can set processor affinity for different programs, but you might not see the huge performance gain you might expect
what major apps do you use or plan to use with this system?