There has been a lot of writing and calculating done on predicting drive train performance with different gear ratios in the FRC environment.
Rather than writing a lot about it here, I'll give you a link to my current favorite "white paper" on the topic
http://www.chiefdelphi.com/media/papers/2750?
You'll find a write up explaining some of the factors that go into drivetrain modelling and a spreadsheet that will help you play with different gear ratios and motor combinations for getting from point A to point B as quickly as possible.
There are simpler spreadsheets available, and it takes a bit of playing with this one to figure out how it all works... particularly if you haven't done much drivetrain modelling, but the write up attached to it... particularly the last page, is excellent.
Short rule, though... for 6" wheels my favorite single-speed reduction has been 14:1. As the paper points out in most FRC applications accelleration is more important than peak speed.
Jason