if you'd asked 6 months ago I'd have said AMD, but there chips are starting to fall behind the Pentium 4 which is good to be pushed up towards 10ghz.
The top end chips are the 2.66 and 2.8, but they are still expensive. The best performance/price chip at the moment is the P4 2.4, selling at around $195 here in the USA. This comes in 2 versions to support different motherboard speeds. The newest speed is the 533Mhz FSB.
Newer motherboards which support the 533Mhz FSB also support faster memory, up to PC3100 DDR. PC3100 is thin on the ground but available if you look. It's a bit expensive but makes the most of the CPU. You can economise by getting PC2700 DDR.
That will at least future proof you up to the 3.6Ghz CPU which is due out in the second half of 2003.
At that stage Intel bring out a new chip, codenamed "Prescott", with a .09 micron die in place of the present .13 micron. They may continue to produce faster P4s however, as they presently do for the Celeron, alongside the new chip.
So, A P4 2.4Mhz, 533 FSB machine with 256Mb of PC3100.