I recently made a vex drive train that had 4 independently driven wheel. This allows it to have multiple drive modes because the wheels can be programmed to be at whatever angles you wish. I created 4 drive modes based off of a paper in
1640's wiki.
You can see a video of the robot working here:
http://www.youtube.com/watch?v=s3gb5n_I7vc
I have it set up so I can switch between Twitch(tank), crab, automobile, and snake modes on the fly.
I would strongly recommend reading team 1640's Wiki and
this paper before deciding on how you are going to power the wheels.