If you're an Ubuntu user, you've probably come across problems when resizing Windows in the recent versions of Ubuntu. Jeff made me excited today by showing me one way to fix this problem. I looked around a bit on the web, and I found an even better way.
To modify Ambiance to have a wider margin, open /usr/share/themes/Ambiance/metacity-1/metacity-theme-1.xml and increase the values of the following properties:
Hopefully you'll find this useful!