I have an Athlon XP 2700+ Thoroughbred CPU and I love it, it performs well for me and it really depends on if what you are doing utilizes the cache a lot or not.
The 2700+ is 2.167GHz (Thoroughbred)
The 2800+ is 2.086GHz (Barton)
Not sure what the 2800+ can do overclock wise, but I know I have my 2700+ running at 2.41GHz without a hitch. (yes, it's unlocked from the factory)
If you need any other information regarding the Athlon XP's with respect to specifications, you can referecen the FAQ I wrote on the processor line.
faq602-4141
Cheers!