The short answer:
For a smaller wood and plastic cutting machine (and maybe – maybe! – some light aluminum), several people on CNCzone seems to have had success with 100-200 oz-in (0.7-1.4 Nm) stepper motors direct driving a 5 turns/inch (5 mm lead) ACME (trapezoidal) screw.
The long answer:
... hopefully you can find in the attached file. Please, if you spot any errors or have trouble viewing the file (like missing characters or fonts), write a comment in this thread: http://cnczone.com/forums/showthread.php?t=5639, or use the email address in the file to notify me. Other comments are also welcome to my email address.