Also, the Pentium M runs much cooler and allows for MUCH better battery life than a P4, celeron or Athlon64.
The P-M runs at lower clock speeds than the P4, so a good rule of thumb is to multiply the clock speed of the P-M by 1.7 in order to get the equivalent P4 speed. So, for example, a 1.73ghz P-M has performance about the same as a 3ghz P4.
The present price-performance sweet spot is currently the P-M 750, which runs at 1.86ghz. A price drop is due soon (the 24th of this month), so if you can wait a week, you might be able to save some money on the price drop, or get more performance, as the new sweet spot should move up to the 2.0ghz P-M 760.
This is a quick breakdown of the P-M numbering scheme:
There is also a series of P-M ending in 5 (725, 735, etc). These are not newer models, these are the older models. The 725 runs at 1.6, the 735 at 1.7, the 745 at 1.8, 755 at 2.0, 765 at 2.1. The difference between the 7x5 and their similarly-clocked 7x0 counterparts is that the 7x5 ones have a 400mhz front side bus, and the7x0 has a 533 mhz FSB. Don't be concerned about it, as the difference in real-life performance is negligible.