A great summary of many of the drive train fundamentals you are asking about can be found here:
http://www.chiefdelphi.com/media/papers/2429
I am a huge proponent of making reliability the number one priority in drive system design. It simply must work. This is why I like direct drive live axles. If every chain or belt on the robot broke, the thing would still move. That being said, I would highly encourage you to consider the possibility of direct driving 2 wheels on a 6 or 8 wheel drop center drive train, or even consider the possibility of direct driving the 4 dropped wheels of an 8 wheel drop center drive. The remaining wheels can be chain or belt driven to gain the advantage of all wheels being driven, but you still get the reliability from the dropped wheels being direct driven from the gearboxes.
One thing that others have not yet touched on is the true advantage of shifting in an FRC drive train. While you had noted that you would like a high gear and a low gear, it is important to understand
why this is desirable in order to execute well.
With just about any single speed 4 CIM FRC tank drive, there is a tradeoff between the abuse the motors see and a high end speed. You could gear for 4 CIMs to never see more than 40 Amps per motor, but you would only go 6 feet per second. You could also gear 4 CIMs to travel 15 feet per second, but the motors might see upwards of 100 Amps a piece at times.
A shifting drive train allows you to get the best of both worlds: rather than settling for an in between gearing where the desire for a high top speed cannot be satisfied without sacrificing motor "health", both of these goals can be met.
All this being said, with many teams using 6 CIMs in their drive train, the availability of miniCIMs, and the possibility of game design completely changing drive train design criteria (2009...), there are ways that drive train performance can be improved without shifting.
When designing your drive train, keep in mind the reasons for using a shifting gearbox in the first place so you can reap the rewards of your work. Consider gearing the low gear such that the CIMs pull no more than 40 Amps at the traction limit. Figure out what your top speed should be for high gear based on your strategy for the game and an understanding of the inevitable trade off between acceleration and speed.