5 Speed to 6 Speed

Hmmm, ok, here we go guys. I currently have a 2002 Celica GT with a 5 speed. I have to drive 40 miles each way to work and most of it is highway that I typically drive at 80mph. At 80mph and in 5th gear my car is at about 4000 rpm's. I love the car to death btw. I was wondering what it would take to change the car from a 5 speed to a 6 speed and if I did, how much would the rpm's drop at 80 if I did so. I am not even sure this can be done but I thought I would ask you guys and see.

BTW, if any of you guys out there with a GTS model with a 6 speed can tell me what your rpm's are at 60 and 80 mph I would appreciate it. My car has the 1.8L engine, so it would make sence to have numbers from the same size engine.....thx

Thx
Rock