For theoretical gear/rear end/speed results I've found the following calculator to be a lot of help. Remember, these are theoretical values, your results may vary, but it's a handy starting point for comparisons.
www.torinocobra.com/gearratio.xls.
It has macros in it so you'll have to enable them if you've got them turned off or set to warn if they're present but it's a clean program.
DonC