
According to the motherboard manual for one of the PC's here (Gigabyte's GA-7zxe, I'm pretty sure; I'll come back and edit this when I get the manual out of the car, if my memory slipped again), I can run up through an XP 2100, not a "+", it seems. But it's the earlier model with 0.18 tracings, not the more current 0.13 version.
So far, I can't tell what the difference is between Model 6 and 8, and/or whether either of those are still in the retail channel. That system currently has a "T-Bird" 1.333 in it, and could use a little more juice.