If you use 2 of the KOP gearboxes in tandem, they are designed to handle the load of 1 chip motor.
This "double stack" will give you a reduction of:
14/50
14/50
14/50
14/50
= 1:162ish
From here you are a simple #35 chain reduction away from 500:1
(10:30 in chain gives you 488:1 overall)
Disclaimer, this setup can handle ONE CHIP MOTOR ONLY. If you run 2, you are toeing the line. But... honestly... with one chip - the stall load on a 4 foot arm is 130 lbs. Do you really need any more torque?
For more information on the "double stack" transmission, refer to the "FIRST 2005 Transmission Manual-RevB" found here:
http://www.ifirobotics.com/kitbot.shtml
Good Luck -- feel free to ask questions about this feedback,
Thanks,
John