The P4 2.2 would be the most powerfull, especially considering that you should be able to oc it to 2.5+Ghz, probably 2.7Ghz! But it would be the most costly by far, and you won't be able to reuse the board or rd-ram with a future upgrade because of the things I mentioned in my other posts.
The Athlon XP solution would be more future-proof (and quite a bit cheaper) as Athlon will continue to use a 133Mhz fsb. I know there has been rumors that AMD will got to a 166 fsb sometime this year, which would give a much increased performance, but that's only RUMORS! AMD has been saying for the last 1½ year that all future Athlons (no matter what core) will be 133Mhz fsb. There is a recent thread in "Cpu/processors and Overclocking" about this, you can find it if you need more info.