Get whatever is the cheapest. I only got the P-M 760 because it was a $50 upgrade from the 750, which was a free upgade from the 730. If it hadn't been a $50 upgrade, and had been the original $250 you can bet I'd have a 1.73Ghz in my laptop right now. I'm happy with my 2.0Ghz though, it's pretty quick. Supposedly the 2.0Ghz is equal to a 3.4 or 3.6Ghz P4, I don't remember which. The 2.13 is equal to the next step up and the same with the 2.26, so it's probably 760=3.4Ghz, 770=3.6Ghz and 780=3.8Ghz.
Anyway, even a 1.73Ghz P-M is plenty for the average user. Unless you're doing a bunch of cpu intensive stuff, like encoding or CAD or something like that, I wouldn't worry about the cpu much.