5spd v. 6spd

Does adding the extra gear help for acceleration, or would the added shifting slow anything down?

and the big question, would a 6spd tranny fit on a 4th generation maxima?

Driving a 6-speed is just like driving a 5-speed except slightly shorter shifting intervals and an extra highway gear.

To answer your second question, no. The 6-speed is designed to be coupled with the 3.5 liter engine and to the drivetrain of that car.