I'm sorry if I was unclear. What I am looking for at this juncture are motors, but I don't know what kind of torque to look for (as torque seems to be a defining factor, and at the moment I am not overly concerned with power consumption).
The casters I mentioned could be something like this: http://www.sparkfun.com/commerce/product_info.php?products_id=320
Just to throw out some numbers, I suppose it could be assumed that I want to use 2'' wheels and that a movement rate of 60 RPM is ideal.
If there isn't a simple equation somewhere that will give me the nice number I want, I may have to go with your idea jack (thanks!).
You must have finished crafting that just as I made my post, which seems to answer some of the assumptions you made. Thank you very much for those equations! This will be very helpful!