As Skuzzy said, this is really too complicated for a simple answer. There are 4 different types of P4s that have now been released: Willamette (1.3 - 2.0 GHz), Northwood 'A', 'B', and 'C'. (The 'C' type is brand new and are the 800 MHz FSB parts.)
Unfortunately I haven't had any firsthand experience with a 'C' type P4 yet, so I can't comment on them from personal experience.
Here would be how I rate them (take in mind that a LOT of things can throw this comparison clear out the window) with the Athlons on an nForce 2 board and the 'B' type P4s using the 845PE Intel chipset. The new i865 and i875 (Canterwood and Springdale) chipsets improve performance slightly for 'B' type P4s and are the only chipsets to officially support the 'C' type P4s from Intel.
The Willamette type P4s are terrible performers compared to their Northwood brethren. (They use Socket 423 and are mechanically incompatible with the newer Socket 478 all Northwood CPUs use.)
The Northwoods, depending on FSB speed (A = 400 MHz, B = 533 MHz, and C = 800 MHz) all stack up differently vs the Athlon XPs and their ratings.
In general, I'd say the Athlon XPs are based on the 'B' type P4s, and up to a point I think their ratings are fair. For 'A' type P4s I'd say they are roughly equivilant to the Athlon XP two rating steps lower than their clockspeed. (IE Athlon 1600+ = P4 1800 'A') 'A' type P4s are being phased out, but are produced at 1600, 1800, 2000, 2200, 2400, and 2500 MHz.
The 'B' type P4s are available at 2266 MHz, 2400 MHz, 2533 MHz, 2666 MHz, 2800 MHz, and 3066 MHz. On these I'd say the Athlon XP performs equal to the P4 'B' of the same rating up to the XP 2800+ (which I think is the last fair rating). The 3.06 GHz 'B' type P4 added hyperthreading, which really throws a wrench into things in that in certain areas performance improved significantly. Overall I'd say the XP 3000+ should be called a 2900+ and the 3200+ should be the 3000+.
Since I haven't messed with a 'C' type P4 yet, I can't say much about them. If I had to estimate their performance from the benchmarks I've seen, I'd say they are the equal of the Athlon XP one rating higher. 'C' type P4s are now available at 2400, 2600, 2800, 3000, (and very soon) 3200 MHz.
In general what you can gather is that IMO AMDs Athlon XP rating system is valid up to the 2800+ (when compared to a 'B' type P4), and a bit of a stretch beyond that. (3000+ = 2900, 3200+ = 3066 B P4 equiv.) In the high end, the Athlon 3200+ is I'd say a touch faster than the 3.06 B (by a VERY razor thin margin mind you) and the 3.00 C P4 is faster it by a small margin.
As for the 2600+ (2133 MHz clock, 266 MHz FSB, 256kB L2 cache) vs 2500+ (1866 MHz clock, 333 MHz FSB, 512kB L2 cache), I'd go with the 2500+ for a few reasons:
1. While worst case the 2600+ will outperform the 2500+ because of it's higher clock rate, under most circumstances the 2500s larger cache and faster FSB will make up the difference.
2. The 2500+ runs cooler because of the lower clock rate and larger die.
3. The 2500+ is a MUCH better overclocker than the 2600+. Very new production 2500s generally are overclocking to 2.4 GHz and are stable with a 200 MHz FSB. The 2600+ can generally only get to 2.3 GHz or so.
4. The 2500+ costs less than the 2600+, which makes up for the cost difference between PC2100 and PC2700 DDR memory. (The 2500+ needs PC2700 for best performance.)
Out of all the CPUs available today, I'd say the 2500+ Athlon XP is the best bang for the buck. It also overclocks fairly well.
On the Intel front, the 2.4C and 2.6C are also excellent overclockers and generally overclock to around 3.2 to 3.4 GHz on new Springdale i865 and Canterwood i875 boards. (You should use PC3200 memory for best performance and stability when bumping up the FSB, which is still expensive.)
Here's my take on the situation. If I were on a fairly tight budget for a gaming rig, I'd go with an Athlon 2500+ on a new nForce 2 board. If that budget weren't as tight, I'd go with a Springdale (i865) board like the Asus P4P800 and the 2.6C P4. (If you go Intel, be aware that both i865 and i875 boards are very picky about PC3200 memory, be sure to check to see what modules are confirmed to work with them. Hopefully future bios releases will improve this.)
