Designing a gearbox is a wonderful engineering exercise. In fact it's one I use when I want to teach someone the basics of FRC design. To start off, it is never completely sure what motors we will be allowed next year, however the combination you listed would be legal for the 2013 game year. As far as gearbox design, the best tool in the world is jvn design calculator.
http://www.chiefdelphi.com/media/papers/2755
If you're smart you'll most likely figure it out quickly. Among other things this table can show the expected robot speed based on your gearing motors and wheel size. The easiest way to do this is to pick a wheel size, the pick gears from either of these sites
http://www.vexrobotics.com/vexpro/ge...pro-gears.html and
http://wcproducts.net/gears-20-dp/
Both of these have high quality aluminium gears which work well in robot drive trains. As far as what final speed you should aim for. most single speed transmissions are are 12-14 fp/s. Once you've figured what gears you want you can use these tools
http://wcproducts.net/how-to-gears/
To figure out how far apart your gears have to be to mesh correctly. This will allow you to figure out where you need bearing holes on your gearbox plate. After this you would draw up your gearbox plates, shafts to hold the gears as they spin and import cads of the gears (these are available from both vex-pro and WCP in the cad section of their site). Then you simply constrain everything together. This is obviously a simplification of the process of gearbox design, but feel free to ask any questions on here if you don't understand something. Also one more link to a helpful article on the subject.
http://www.frc-designs.com/btd/roboticstutorial1.html
Happy cadding!