I have a 17" MBP 2.33 GHZ with 4 gigs matched pairs ram installed (get 3.3GB performance wise). I have it connected to a 22" monitor, and am able to use Illustrator for logo design quite nicely, tyvm. I'm also using CS3, but have Tiger installed, and find it does not lag at all, and is quite efficient. BTW, I have 7200RPM drive installed, and an external G-tech fw 800. The current MBP would actually meet and exceed your needs for logo design.
I think the 15" MBP would be fine only if you intend to attach and external monitor. You could probably get by with a 22" for logos. I agree that Illustrator CS3 is the program of choice, and if you have long term goals as a logo designer, you might as well start with an industry standard program, rather than play around with the toys.
I don't have a MB, so I'm not sure if you could get by with one, but I suspect you could if you use a 22" external monitor. Again, if this is a long term interest, you will have to weigh your finances against your future, and decide if you want to get the less powerful laptop now, or go for the one that will be there for you when you grow. Either way, it's probable that you'll be upgrading as you grow anyway, so just some things to consider.
Since you haven't mentioned going into heavy 3d rendering, or large print/photo editing work, I think the above will suit your needs with logos. BTW, a 24" iMac would also do a bang up job for logo designing. Good luck to you.
