The desktop that I use for software development is an Apple Mac Mini. It's mostly an X window server for a couple of Linux systems that are my build and repository servers. Below you can see the tiny silver Mac Mini sitting on a shelf above the Cinema Display.
Here are the servers in the basement, next to a spare LCD display, a honking big H-P full duplex color laser printer, and two two-drawer fire-rated filing cabinets. My backup strategy is as follows: I backup my two principle laptops, a MacBook Air and a ThinkPad X61t, to USB disk drives; I backup the two servers by doing disk-to-disk copies to removable disk drives; I do smaller incremental backups to media like CD and DVD ROMs and USB thumb drives; I also keep backups of individual projects on an off-site server.
The backups are kept in one of the fire safe drawers. When I travel, any of the laptops that don't travel with me also go into the drawer. The ThinkPad actually fits in a file folder. The Air doesn't quite fit, but there is enough clearance for it to just sit length-wise on top of the file folders.
On a recent trip I was just taking my Blackberry and my iPad. Both laptops went into the safe. I was regretting not having done a recent backup of my desktop system, when I realized I was thinking about this all wrong. The Mac Mini is small. Really small. File folder small.
All of the connectors on the business end of the Mini disconnect easily.
This makes it trivial to shutdown my desktop, disconnect it from the network, display, and USB cables, and place the entire desktop system in a file folder in the fire safe, leaving the cables velcro-tied together and just dangling.
Good luck doing that with a standard-sized white box PC cabinet.
Smaller is better.