[Matt Krass]

I agree with Jamie, sensors, sensors, sensors, very important, for your robot to do anything more intelligent than blindly firing motors it needs information. Also, shameless plug, in my signature is a paper on PID control theory, and lots of examples, and common pitfalls and some tricks, let me know if you use it and if its any help!

In reply to kaszeta's post, what you've described is a P loop only, there is no ID control in that, see the formulas in my paper for further examples of that control.
With just P you're either going to under or overshoot a lot, and it will probably slip more because small errors won't generate a significant output. Also I would never ever recommend starting with such a high gain, you're just asking for something to snap around at full speed, but you did remember to include a speed limiter sanity check after, so it's not really a problem, but you won't see anything other than 64 in either direction until you have a really tiny error, which means more than likely an overshoot.