The rating of 2600 Barton xp and 2500 xp are way off compared to 2600 Throughberd. Barton has only a clocked speed of 1.9 ghz compared to the 2.083 ghz speed of the TB. In most case the extra 256kb of cache isn't going to make up for the 183 mhz clock difference at all! The 2500 rating is even worse. Its a whole 283 mhz behind and cache again can compensate for it all. It needs to take much bigger drop than mere 100 PR rating. Only way barton could be better is you OC it! AMD made bad mistake rating those 2 things.
1) there is no barton xp2600+. There are 2 xp2600 chips, 1 is 15.5 X 133, and this is the old, hard to find model. The other is an 11.5 X 166 chip. This is the new thorton. It gains perf ( remember that is what the xp rating system is based upon ) from the faster fsb. The xp2500 is also a 166 fsb part, but runs at 11 times. It gains some of it's score from the extra 256 cache.
The main problem with improving cache is that it gains in some places and it doesn't. If you look at the A64 3000 compared to the A64 3200, there isn't much differene in performance even though 3200 has double the cache. Cache can make things fall behind in some place and take ahead in others. Take look at this recent review from anadtech with the 2600 troughbred and 2500 barton:
<A HREF="http://www.anandtech.com/cpu/showdoc.html?i=1927" target="_new">http://www.anandtech.com/cpu/showdoc.html?i=1927</A>.
I will admit the barton kills 2600 in some places (IE games and business), but in others it fall behind the 2400 xp! You could say it balance out evenly, but old throughbred B 2600 is really more balanced. It doesn't fall behind that much. AMD trying justify things with cache results in stupid stuff like 3200+ barton with sucks. OC a 2500+ to 3200+ is what most ppl do instead of blowing $$ on 3200. Anyhow, just justifying things in cache isn't the best way to go!