Don't confuse MHz with power. That is 2 year old marketing still working on your mind. Its all about the architecture. The Pentium-M has in many ways a superior architecture to the P-4. The P4 has really long pipelines - the P-M has short ones and is way more efficient with its branch predictions.
Here's an over-simplified way of thinking of it. The P4 has a really long hallway, that at the end has a room you need to hand a paper to a guy, but you don't know which paper. So while you can run real fast down the hall, if you predict the wrong piece of paper, you essentially loose that cycle.
With the P-M, the hallway is shorter, and you can predict the paper he wants better. So although you run slower, you can complete more trips correctly.
A64s do this process the best, but they are hot, large, and power hungry. P4s are hot, large, and power hungry. P-Ms are cool, not power hungry, and pretty darn good.
You need to decide how heavy is too heavy (a Dell top of the line weights almost 10 pounds, but its a bruser of a workhorse), cost, and performance. Nobody can decide this but you.
Just make sure you get at least an sxga screen doing those things and a gig of ram. However, if you are using things like Reason or any MIDI programs, there are reports that the new 533 MHZ bus Centrino2's have major issues with sync.