This is what I have based my machine on. It is a shopbot. It is driven along by rack and pinion. I have used timing belts instead. If you want I can show you some pics of my Bennybot machine.
here
The main difference between mine and shopbot is mine uses servos, timing belts, and is driven from both sides using a conecting drive shaft. Besides that the shopbot looks a whole lot nicer than mine.