That is the Thoroughbred (T-Bred) which is built upon the .13 micron process. The Thunderbird (T-bird) is an older version, built upon the .18 micron process.
Keep in mind the T-bred and T-Bird are two different processors. And even then, there were 2 revisions of the Thoroughgbred.
The T-bred everyone is crazy about is the AthlonXP 2100+ Thoroughbred-B. That is because of the overclockability of these things. I managed to hit 2.4 ghz (real ghz, no PR crap), around a 3000+ rating from AMD. There are several people who even have higher OCs than I have achieved.
If you are looking to overclock, look at the AthlonXP 2100+ Tbred, not the retail.
I hope I clarified a little better.
If you have any more questions, don't hestitate to ask.
Jonathan