As a former G5 iMac owner and current satisfied customer of an Intel Core 2 Duo Macbook, I can say that the speed of the current crop of laptops, even at the "low" end, is so good that my Macbook easily outpaces my older iMac and even the first-gen Intel iMacs, IMHO.
It's a lot of computer for the money. You can take it with you and buy a good mouse and 24" monitor to dock to with the money you save.
I can't speak for the current 24" iMac, but I'd think twice before buying such a large computer (if something so self-contained can already actually be called "large"). If I needed more raw processing power than a Macbook, I'd hold out for a Mac Pro.
AAM: When Should I Buy?