Actually you don't really need a VGA to BNC converter if all you want to do is use the same monitor and switch between computers. I'm on such a setup right now. Just buy a Belkin switch box -- it has two VGA inputs and one VGA output to the monitor, and an A/B switch. Simple. Costs about $10.
They also make ones that switch not only the monitor but also the mouse and keyboard, so you can save desktop space. That's what I have, to switch between my Win2000 and Win98 systems.
Check it out:
http://catalog.belkin.com/IWCatSecti...s?Section_Id=5