Mac OS X or Mac OS 9.x?
Both systems ship with basic hard drive formatting/partitioning applications you can use.
Typically, since you can't repartition an active drive you'll need to boot off the System CD and run the disk utility to format and partition the drive (after backing up your important files, of course).
Once partitioned you'll need to reinstall the OS, applications and data, etc.
In my opinion, it's not worth the bother. While the idea of having a partition in sync with the iPod is nice, what happens if you upgrade the iPod to the 10gb version?
If what you're most concerned about is not being able to put more on the partition than you could fit on the iPod, a simple Folder Action script could alert you to when you're approaching the limit.