I've written a document that attempts to explain how to use the motor spec sheet and do the appropriate calculations:
http://www.botsnlinux.net/firstphysics.php. There are also a lot of other great resources here on CD about selecting the right motors and doing force/torque calculations. Search around, especially in the CD-Media white paper section.
Finally, don't just decide based on your gut or what people here say. Do the math and be sure for yourself!