These threads might help a little. They're physics quizzes from ether (both of which I "won", may I add). The first tells you the torque (applied to the wheel) required to start a square, 4wd, robot rotating. The second is the same problem, but for all rectangular robots:
http://www.chiefdelphi.com/forums/sh...ad.php?t=98947
http://www.chiefdelphi.com/forums/sh...ad.php?t=98971
The result for the second is that tau=muNr/sqrt(f^2+1) where f is trackwidth over wheelbase. You can see that for a square bot, f equals 1, which yields the solution to ether's Quiz 5. A narrow bot (f<1) requires more torque and a wide bot (f>1) requires less torque. You can apply the same principles to other tank drive setups, too.
As for tippiness, changing your trackwidth will give you no boons. All the tipping forces are in the plane formed by the vertical direction and forward direction. A two-dimensional robot will act the same as a three-dimensional robot that is infinitely wide in this case.
If you want to go wide, go all the way.
Math is always fun, especially when it's useful. - Guy Davidson [more] 
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