A wealth of good information here for sure but let me throw in my opinion of running 6s vs 5s.....I have learned that running 5s seems to be much more efficient due to the high RPM jump when going with 6s.......the very first thing you should do is learn to use BrianG's speed calculator
http://scriptasylum.com/rc_speed/_top_speed.html it is the key to dialing in any truck.....you can get just as much speed out of a 5s truck (with the exception of speed runs) and run at a much lower rpm ...hence less fatigue and heat on the motor.....let the gearing do the work
LEARN TO USE THE CALCULATOR