see my site for what i did:
http://dannygalaga.com/mame.htmi used a tamiya planetary gear-box kit ($27 Australian, should be less than $20 USD). I used a 1/12 scale r/c wheel to drive the monitor base plate. this is on a 15" in a cocktail though. The total ratio is somewhere in the region of 2000/1 which rotates through 90 degrees in less than three secs. Good thing about this method is that if something goes wrong, the drive wheel will just slip instead of the motor stalling and burning out. I use an on/on toggle to activate. powered by the speaker transformer. No need for relays or anything.I d love to hear from anyone who has theirs switched by software!
larger monitors in an upright i would suggest hacking an electric screwdriver. theyve plenty enough torque.
Also, where I have stop switches for the motor, i also have switches to turn the monitor on and off. being a 15" pc monitor it normally degausses in the three seconds or less time it is off. But i will add a relay with a capacitor to delay it switching on for another second after it stops moving. Commando in particular seems susceptable to problems- the corners go purple.
here is a GIF of the beast. for a short movie of it in action go to the bottom of the 'craftwork' section on my site.