The most usefull method would probably be through a tv card, monitors run off of RGB and atari (and all other game systems) run off of composite (which is basically S-Video)
in order to convert Composite to RGB you need an Up Converter which can be found online for around us$250 (too pricey for my blood) although it would give the best picture and with an input selector you could hook up many game systems for hi-res graphics.
so it wouldf look like this:
game system1---
game system2-----input switch----Up converter----monitor
game system3---/
another thing you could use with this setup is an A/B switch between the converter and the monitor you that you could still use the monitor with your computer
I know that you can find input switches at Wal-mart for about us$20 (although it had the names for the current game systems written on the buttons) and an A/B switch can be found online at http://www.cablesnmor.com/goto/video-switch-box.html
without the converter the setup would only cost around us$30 but the converter is the expensive part... I know there used to be one called a Cheese Box that was around us$69
anyhow, hope this helps, as you can tell I've thought about all this before