Picking the "right" hard drive size is getting trickier every day. My first impulse would be to bite the bullet and order your MacBook with the largest hard drive option available.
With all the digital media out there now (movies, music, photos, etc etc), you can fill up so much space so quickly. For example, a few TV show seasons in iTunes or ripped yourself, will quickly eat up a huge chunk of a 120 or 160 GB drive. Keeping 20GB marked off for Boot Camp (which for most of us is secondary to media storage) further limits the space you have for goodies.
Leopard's Time Machine creates the dilemma here. The larger the main drive, the larger the external backup drive you will need for Time Machine. Of course, this makes backing up goodies simple but only if they all fit on your main drive. Once you start storing them on an archive/secondary drive, they no longer get backed up with Time Machine. Which means, of course, you would need a *third* (yikes!) drive to backup your archived media drive.
My current solution is a portable drive for media goodies (so I can keep a healthy amount of disk space on my MacBook free), and a large external drive partitioned in half, 1 partition for Time Machine and 1 partition as a backup of my portable media drive.
Hope this makes sense? The short answer: get at least the 160GB -- for a mere $75 more, you can't go wrong. Thankfully the MacBook, unlike its predecessor, can easily receive a hard drive upgrade later.
--Chris