Overall, Linux is not known as a resource hog. The free operating system is a fairly lean machine out of the box -- some distributions moreso than others. Still, there are some tweaks you can make to any Linux installation to speed things up.
The problem with speeding up your desktop is that you have to make certain usability compromises. I had to make my girlfriend's desktop fast, yet as user-friendly as the GNOME environment she uses on her own Ubuntu-based computer at home.