it does depend on the depth of the unit itself quite a bit. obviously some of the double dins with all the bells and whistles will need much more room than standalone screens or units with less features (like built in hard drives/etc)

 

just chiming in with my .02, i would say a stand alone would be great, but you would have GPS under one seat, DVD, under another, and CD player in stock location...sounds like way too much work, wiring, and too many seperate parts. i would recommend that you go with a CarPC with a standalone monitor in dash. no clearance issues, and you get some functionality with it too. only thing is, is finding a place to put a cd/dvd drive...which would fit under the drivers seat and the tray could pull out to the area between where your feet would be if they were that close.