Want a simple (but not too simple) robot project, using easily available, off the shelf components? Try checking out Overview – HollerBot: Open Internet Robot. The idea is to spend under $500, and get a robot with motion, vision, gps and all sorts of other goodies. Seems like a good way to go.
Addendum: I would be tempted to use one of these in the robot instead, or perhaps this one (fanless). Why? Includes a CompactFlash slot and PCMCIA. Seems useful. I don’t think having a hard disk on something with wheels is a great idea.