Having run LInux (and X more specifically) on desktops and laptops for a long time, I have tried a lot of different configurations and setups. Recently however I found myself with a need for two workstations and while I had two sets of keyboards, 2 mice, and two monitors, I had only one PC (running Debian unstable). It took some trial and error but running multiple desktops from one not-so-new PC is both relatively easy to configure and surprisingly usable once set up.