Remember, On or Before is On or Before

So don't feel too bad yet; if its 2/7 you can begin to be upset for waiting a bit. But in reality, Apple, like Dell and everyone else, tries to make a worse case scenario and tries to ship before that date.
I would have to think that will be a bit slower. I've got a Pentium M, which smokes the desktop Pentium 4. If you have the Pentium M in your notebook, then it will definately be faster. If its really a Pentium 4, they will be closer in speed; the G4 may be faster in multimedia. Overall the general responsiveness of the Mac will probably seem the same or faster just because all the elements of the UI are accelerated.