A six volt pack will give you more powerful servo torque but less flight time. Sad but true. You need a 4.8V pack with more maH rating. I switched to 1450 maH Rx and 1650 maH Tx NiMH packs in my models and transmitters and haven't looked back. Cost a little more, but add two flights per pack.

Check out Red Scholefield's battery clinic for some great info.

http://www.rcbatteryclinic.com/