The standard HiTec servo is a 311. This is a NON ball bearing servo and has 51 oz of torque. The 325 BB is a ball beariing servo and it also has 51 oz torque.

For MOST applications in a 40 - 60 size plane these would be fine. What you need to do to be certain you have the right servos is to use this calculator.

http://www.csd.net/~cgadd/eflight/calcs_servo.htm