You are, essentially, building another remote control to the one you already have.

So quite apart from the obvious, Laptop and Joystick, you will need some way to read the joystick and code it into commands then send it out the serial port. Have a look at http://www.roborealm.com for this, makes the job much simpler.

From the serial port you are going to need to send it to the plane, something like the XTend or XBee modems should do the trick for you.

In the plane you need a microprocessor board to connect the modem to and multiplex you Rx signals and autopilot signals to the servos.
Have a look at http://www.millswoodeng.com.au

Then you will need an autopilot to interpret the commands and turn them into servo pulses. Something like the Ardupilot might do the job here, at least it is cheap at only $25US, check out http://www.diydrones.com. They also have some opensource code that will get you started and it already handles stabilisation, which is another aspect you will need to look at.

I trust this is enough info to get you started.

Mike