First off, you can customize your Desktop Environment (Ubuntu uses Gnome, I believe) to great extents. Our FAQ on this has some good information on places to look:
http://www.neowin.net/forum/index.php?showtopic=148258Plus, you can also completely switch Desktop Environments, or Window Managers. You can use KDE (on par with Gnome), or XFCE (lighter on resources than KDE or Gnome). You can even switch to very light Window Managers, such as fluxbox and others.
Each of these can be customized. There is literally
no limits to the customizibility (is that even a word?) of a Linux box.
I would suggest looking at customizing your existing
Gnome DE first. Then, if that isn't enough, start looking at other DEs or WMs.