being the user of a 1.67 GHz 15", I can say that with enough ram in the machine, you'll see performance at the level of a 2.0 to 2.2 GHz Pentium M or so in "everyday" work.
but it really all depends on what you're doing, the processors are better at different tasks. for instance, running photoshop filters on a 3.0 GHz Pentium in, say, a Dell XPS laptop, will feel like molasses compared to a 1.67 PowerBook, simply due to the G4 and AltiVec's like for vector operations.
but remeber, most of all, you're on Mac OS X. everything will "feel" different compared to a PC laptop running windows or linux, and that's all you really need to know.