Buy an HDMI-capable monitor and then buy one of these: http://www.amazon.com/Ultra-High-Performance-Switcher-Switch/dp/B0015YWKYY/ref=sr_1_1?ie=UTF8&qid=1355319901&sr=8-1&keywords=hdmi+switch
I use a similar product on my home TV, which has two HDMI inputs for my three devices. Cable box goes into one HDMI port, and my 360 and Bluray go into the switch which goes into the second input. You just hit a button on the switch to go from one input to the other.
The only caveat is that the switch uses intelligent switching, so when you turn on the second PC, it will automatically detect that and switch to the latest input. Easy enough to switch back, but something to think about. If you look for other switches you may be able to find ones perfectly suited to your needs.