AFAIK, the xp-m runs at the same speed as the 2800 bartons, but uses a 133/266 fsb, while the standard 2800 uses a 166/333 fsb. To the best of my knowledge, the xp-m 2800+ is a barton part.
Some people are selling the socketA sempron-m as an xp-m part. I think they have the reduced cache, like the t-breds.
I just realized this now. Bought today a bundle from ECS KM400-M2 rev 3.0 with an athlon xp-m 2800+ and I can't get to work past 6x 133mhz (800MHZ) Why the hell are they dumb enough to sell a bundle like this if you can't alter the multiplier??? For now I'm using Rightmark CPU clock utility to force it to work at 16x. Don't know if it's the same as doing on the mobo
this is the specs I got on the net. I guess it's what I have.
Athlon XP-M 2800+ 2133 MHz 512 KB 133 MHz 16x 1.65 Barton AXMA2800FKT4C