I'm eventually going to but an AMD Athlon XP chip, somewhere around the 2600 model, but some models are made with both core types.

Do I even need to worry about it?

If the core type is not listed in the detailed description of the CPU, then how do I tell which core the CPU has?

its my understanding that AMD used the **00+ model series with the claim; this number was that Athlon XP chips comparability to a Pentium 4 chip running at that speed. In other words, An AMD Athlon XP (Thoroughbred) 2600+ with a clock speed of 2.09GHz (384kb cache)will do the same work as an Intel Pentium 4 chip with a clock speed of 2.6GHz (512kb cache) because of a more proficient architecture.

"The major difference is that the “Barton” core nearly doubles the L2 cache from 256k to 512k for a total of 640K cache." - AMD
does the Barton with the newer core and the more cache with a clock speed of 1.83GHz do more work than the Thoroughbred with a clock speed of 2.09GHz? I dont know?
I have a Thoroughbred 2600+ and my buddy has a Barton 2500+ but are PC's differ in other areas so it is impossible to make a comparison between them. I could throw his chip in my PC but considering I have a GigaByte (1024MB) of RAM and 2 Serial ATA hard drives using RAID on a 333MHz bus, could you tell the difference? me neither.

However, I dont think either of our chips has the latest .13 micron architecture that AMD XP chips now ship with. I got my girlfriend an 1800+ .13micron but I dont know if that is the Barton or Thoroughbred core?

Looking around on www.pricewatch.com I had trouble finding an actual sellers site that claimed .13 and 2600+. newegg.com and xtremegear.com are great stores to buy from but they claimed .13 on the pricewatch ad and not on the actual site you order it from.
the best I could do was here...
http://www.upgradecenterinc.com/amdatxp28bap.html
would I prefer to have that chip instead of what I have now? I don't know, but I must say I am very happy with what I have.

www.amd.com is a hard site to find what you are looking for but they do have an inter-active support page. you could ask them and wait for an email. they would be the ones to know...
they also have forums now so you may want to post your question there.

here is all three lines of that address:
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http://139.95.253.214/SRVS/CGI-BIN/WEBCGI.EXE?New,Kb=AMD_TSC,Company={ECE6A021-EAA1-427C-BB74-20EB3D8A595C}
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there is a difference in performance you will get a little more performance with the barton core then the thoughbred core the L2 coche give the cpu more performance lets us the 2800+ chip because it come in barton and throughbred cores the barton core with the more coche has more performance but least mhz then the throughbred core and it has a little more performance the throughbred has more mhz but it has less performance then the barton but not by much i hope this helps

It's either going to be a Thoroughbred B or a Barton. The Bartons are available in 2500+, 2800+, 3000+, and 3200+ currently. Thoroughbred B's are everything else. Companies do and should denote if the processor has a Barton core or not.

Kevin,

It doesn't "nearly double" the L2 cache, it does double it.

And the whole reasoning behind AMD's PR rating system is that they benchmark the processors against themselves. It has absolutely nothing to do with Intel, and I can guarantee that if you ask about it, the explanation you gave is what everyone else will say, but I'll argue it until I'm blue in the face. AMD has to compete against Intel yes, but they don't rate their processors against each other. The PR rating of 2600+ means that processor is that much faster than an Athlon 1 GHz, not equal in IPC/clock speed to a P4 2.6GHz.

Your girlfriend's 1800+ is probably a .13 micron Thoroughbred "B" (B denotes .13 micron), your buddy's Barton 2500+ definitely is, and your 2600+ most certainly is.

It's all semantics otherwise.