It is as you make it
 

The really cool thing about Linux is your choice. You have a lot. The important things to remember are to
1. Learn 2. Explore and seek answers, 3. Play and have fun, 4.work the way you play

With that out of the way, there are several areas I will get you to explore to get your computer looking geeky:
1. Window Manager. KDE, Gnome, Fluxbox, Enlightenment and the others have been listed I can personally speak from (k)Ubuntu,Debian and Mint that it is easy to install them all on your computer if you choose.
2. Window Themes (if you Window Manager supports them)
3. Icons
4. wallpaper
5. Compiz for the extra flash
6. Splash Screen
7. Color scheme

As I said, explore, learn, play. That is the key to mastering Linux.

Don't forget widget toolkit (GTK+ and Qt) themes.