Another point is that the new mobo will most likely force you to also replace your SDRAM with DDR memory...
There are some boards on the market that have both types of slots...stay as far away from them as possible. They're crap.
As for the "duallies on a uniproc board" question...where would you conceivably put the second chip?
you need a second socket.
The Athlon MP and Opteron are dual-capable, the Athlon XP isn't.
Ax's statement rings true. unless you're doing high end graphics tasks or back-end server computing, it's not worth the cost of the more expensive cpu's.