Check the CPU Database at overclockers.com for answers on overclocking potential for the 2100 Palamino. I think sort of as a rule of thumb, as the CPU speed increases, the Percent overclock you can expect goes down for a given Core type. There is a limit to what the core is capable of. The XP2100+ was the last of the Palaminos for a reason.
The Process had about reached it's limit. The 2100+ will overclock, but the percentage increase will be smaller than a slightly slower CPU because the slower CPU has more headroom before it hits the Core's limit. That's why AMD changed the core to the "A" type and again to the "B". I doubt you'll find any "A" type 2100+'s yet. They will almost certainly be Palaminos. Best to wait a few months and see if you can snag an "A" T-bred 2100+ although they seem to be tricky to overclock if the 2200 is any indicator. After all the "A" type only adds one more speed grade than the Palamino. Not much more headroom there.
AMD does list the "B" core in 2100+ speed in it's tech docs, but it may be a while until they would trickle down to that speed. Yields will have to improve before they start speedbinning "B"'s down to the lower speeds.
In order, The T-Bred "B" should o/c better than the "A", and the "A" should do better than the Palamino, speed for speed, all else being equal (Never in real Life).