Unfortunately, there's nothing you can do about the hard drive. You just have to determine how much space you want to allot to each OS. Think about a lot, because you can't adjust after it's done. You'll have to do a complete reinstall.
How much memory do you have? Are you installing XP or Vista? If it's XP, whatever you have should be fine. If it's Vista, you need at least 1 gig, but I would recommend 2. I don't know about Macbooks, but on my MBP installation of more memory was very easy. Apple offers instructions on their site under support.