looking for advice regarding robot wheels

The video shows my project thus far:

http://www.youtube.com/watch?v=tN8rNp8Eb8E

I'm thinking I want to use wheels to provide mobility. Looking for advice on what type motors would be best suited for my application. Two Hitec 311 servos should be arriving in the mail tomorrow...not sure whether I should try to modify them for moving this beast or to use them for a pan and tilt setup for the head and look to using stepper motors instead. I'm trying not to spend a pile of money...so may opt to skip the pan and tilt feature...at least for now.

I'm a little confused as to whether I should be powering the arduino with the 9 volt or the 4 double A's. I had it running of the AA's last night and it was giving some strange results...losing power...LEDs flashing when they shouldn't have been. My wiring job is not top notch...and the batteries have been sitting in a closet for a few months but have not been used. Seems to run fine off the 9 volt.

I welcome any input whatsoever!