Any Barton 2500+ will overclock nicely for you, even the locked ones (although you will need good RAM, and a good motherboard to clear 2200 MHz). A pre-week 39 Barton is unlocked - and therefor more desireable. The week takes the form of 03XX in the stepping, where 03 is the year (2003 in the Barton 2500+'s case), and XX is the week - which you want to be less than 39 for a guaranteed unlocked processor.
Thoroughbreds are tricker - as there are several different "Types".
First look for a Thoroughbred "B" core processor - identifieable by the stepping, where the last letter is B. XXXXB - examples being JIUHB, NIUHB, JIXIB, JIXHB, KIUHB.
Next make sure the chip is a DLT3C processor - the L indicates a stock Vcore of 1.5V - you do not want a DUT3C proecessor, as they stock at 1.6V, and are worse overclockers.
After you have found some T-Bred "B" DLT3C processors buy whichever one is newest - again indicated by the week. Be warned though, the very last batch of 1800+ chips is locked, again, like with the Bartons, week 39 and up is undesireable.
An example AMD XP 1800+ Thoroughbred "B" DLT3C Stepping:
AXDA 1800 JIXIB DLT3C 0332 WPMW
And I have bolded the parts that are important. The rest is unimportant, and can be ignored.
