Is an XP a Tbred? Yes and No. 'Thouroughbred' is the new .13 micron core of an XP. Athlon XP is just the name that AMD uses to market their processors, so they end up calling two moderately different processors by the same name.. There are two kinds of Tbred as well. The Tbred 'A' & the Tbred 'B'. Tbred A's run hot and don't overclock very well. Tbred B's run cooler and overclock better (although not much)..
Palomino is the core that the lower speed XPs use, but eventually all speeds will move over to the Tbred B core. Currently it looks like this..
XP 2400+ & up = Thouroughbred B
XP 2200+ = Thouroughbred A
XP 2100+ & below = Palomino
Hope this helps.