[dnanian]

Before you partition, create a sandbox on the full drive to see how large yours ends up being. Note that some files in /Library (Garage Band, iWeb, iDVD) can be quite large, and aren't shared by default, so they take up extra space. If you want to share them you can do so -- see the User's Guide for the technique to use (there's an advanced example that shows how to share additional files).