I use KDE so .. I might not be of much help with Gnome, but I know there are some 'themes' that you can install (they need to be made for Gnome).
KDE can be greatly customized, go to http://themes.kde.org/
and take a look of that desktop manager.
-Themes there usually have some sort of help in how to install them.
- Not all themes modify ALL of your desk top. Some will change a small part
- SuperKaramba is a nice tool to create nifty desktop eye candy. (need to install SuperKaramba and some of it's themes)