Most 550 motors I've seen are 12 or 14.4 V. You'll want to run ANY DC motor as close (or slightly higher) to it's rated voltage as possible. When you run them lower you loose a lot of efficiency.
I'd recomend either running 6 or 7 cells on a 540 or more perferably 12 to 14 cells on a 550 Titan or Johnson motor. The Mild Maxx, etc motors are junk - check out the Traxxas forums. The Mag Mayhem is a good one if you can find it...
