And They Skipped By 133MHz
Now, the big question that lingers in everyone's mind is, what is the "real" clock frequency of the Athlon XP 2600+. While we can extrapolate and predict the clock of the 2600+ based on AMD's TPI (true performance initiative) methods, we realized that AMD has decided to make a bigger 66MHz skip somewhere between 2300+ to 2500+. Since the Athlon XP 2200+ is clocked at 1.8GHz, we would have guessed that the Athlon XP 2600+ should be clocked at 2.06GHz (based on a 66MHz increase in clock frequency for every 100+ increase in model number). However, this is not so. The Athlon XP 2600+ is actually clocked at
2.13GHz! See the table below.
Athlon XP Model Numbers and Its Frequency
Model No. 
Multiplier 
Frequency 
Clock gain over previous model 
1500+ 
10x 
1.33GHz 
 
1600+ 
10.5x 
1.40GHz 
66MHz 
1700+ 
11x 
1.46GHz 
66MHz 
1800+ 
11.5x 
1.53GHz 
66MHz 
1900+ 
12x 
1.60GHz 
66MHz 
2000+ 
12.5x 
1.66GHz 
66MHz 
2100+ 
13x 
1.73GHz 
66MHz 
2200+ 
13.5x 
1.80GHz 
66MHz 
2300+ ** 
14x ** 
1.86GHz ** 
66MHz ** 
2400+ ** 
14.5x ** 
1.93GHz ** 
66MHz ** 
2500+ ** 
15x ** 
2.00GHz ** 
66MHz ** 
2600+ 
16x 
2.13GHz 
133MHz 
** at the time of writing, we have no information of these processors, thus, the numbers here are just pure speculation.
The BIOS screenshot tells us it's a 2.133GHz CPU. Better upgrade your BIOS if you want to upgrade to the Athlon XP 2600+.

HOda's WCPUID screenshot.

If you look at the table above, the multiplier of each processor model increases by 0.5x which roughly equates to about 66MHz of increase in speed for every processor model number. However, if you just assume the next three processor models after the Athlon XP 2200+ to linearly increase at 66MHz steps, you will find that the 2500+ would be clocked at 2.0GHz. Now, what we have here today is actually a processor that is clocked at 2.13GHz and that is like 133MHz increase over the previous model. The multiplier skipped by 1x instead of the tiny 0.5x increase in previous models.
The development of the Athlon XP is rather interesting as AMD seems to realize that it's about time that they should scale their frequency a little faster than they are used to. If you've been following closely, you would find that the increase in performance from each model number is only a small fraction and it would not seem to benefit much to overall performance. Perhaps AMD realized that in order to catch their competitor in this GHz race, they would have to up the ante and what better way to do it with their new Thoroughbred core.
However, the changes in frequency and model numbers does seem to be a catalyst for more unhappy customers, or on the other hand, trouble for AMD. If you look at it this way, the frequency jump for the Athlon XP 2600+ is 133MHz more than the previous model (that is if AMD decided to make that leap between 2500+ or 2600+). So, if you are the customer, logic would tell you to completely skip the 2500+ processor simply because it makes more sense to go for a much faster processor although the model number increase remains transparently constant to the consumer. On the other hand, AMD will not be able to price the 2600+ too high because their processors' value are closely related to their model numbers and not the processor frequency. In our view, it's a dangerous step that AMD is making simply because AMD's model numbering system is now in question.
How about AMD's 512KB Level 2 cache wonder, Barton? Well, when that comes out, it will yet be another big challenge to AMD  especially when it comes to branding the CPU and numbering them. It will be a consumer's nightmare if AMD didn't do it well, but we'll just reserve our comments until they are released (hopefully before we say hello to 2003).