I am running a TigerMP with two 1GHZ Durons, it will take XP's, or MP's too.. You can use the TBirds and they will "work" in SMP but they do not perform well.. For doing SMP you are better off with a pair of (Morgan Core) Durons than the older tbirds of the same speed. the old core was not optimized for SMP.
As far as the 760MPX not allowing tbirds, I don't think anyone has tried.... but there isn't much of point to it anyway.
to get the full AMD Dual processor experience I suggest you check out the motherboard forum on 2cpu.com There is a whole lotta AMD SMP lovin' going on over there..