Personally, I kept a G5 Mac that I run some old Silverfast (CS2 plug-ins) and Nikon scanning software on for my old Epson 4870 photo and Nikon Super Coolscan 4000ED scanners.
I installed a IOGear GCS1762 DVI KVMP switch that switches easily between the 2 computers with the push of a button. Using one display and keyboard/mouse. My main computer is a 15" MBP, with a GB switch for fast transfer of files between the 2 computers. Setup works pretty well, and sometimes having a spare computer has come in handy.