While Linux doesn't have a reputation for eye candy, you can nonetheless get a great desktop with a little bit of work. Compiz will normally be working by default, so with a little effort you can use this to really make your desktop into something special!
If you want to get more out of Compiz, then you need to install the package compizconfig-settings-manager. This gives you an easy-to-use menu which allows you to set your preferences for Compiz, so you can get the famous cube effect going.
For a more attractive desktop, you can install Emerald. This is a theme manager for Compiz, allowing you to use loads of great themes to make it look even nicer.
And finally, if you want an OS X-style dock bar, install the avant-window-navigator and awn-manager packages from the Hardy repositories. With these, you can set up a dock bar and customise it to your heart's content!
It's staggering to see how great some people have made their Ubuntu desktops! If you need more resources, check out Gnome-Look.
Xubuntu will also happily run Compiz, Emerald and Avant Window Navigator, however Kubuntu is a bit less well served. It's possible to get things like Compiz and Emerald working, but it's a bit more of a struggle. But KDE4 has eye candy aplenty, so if you're using that you're OK!