The 2 solutions are a switcher box, or a full blown KVM solution. If you want to also get rid of the extra keyboard and mouse and just have one of each for both systems too then get a KVM. I would however recommend looking to see if you can find one with a manual switch between the 2 systems, as auto ones require a keyboard shortcut, which doesn't always work.

If it was just the monitor you wish to switch then a simple switch will work. These are just a metal box with 3 VGA connectors on the back, and a A/B switch dial on te front. You connect the monitor to the middle VGA connector on the back, and 2 VGA cables to the other 2 ports going to each system.

With these systems, long cables will cause ghosting on the screen, so make sure you use short cables if you can.

You can get cheap KVM setups for 2 systems that have the cables all combined into an all in one molded soluton. They are not too much money, and shouldn't suffer from the ghosting. Belkin make these, as do others.

For the basic switcher boxes, I bought mine from Maplin.