do you need/want your pc in between? you could just hook a joystick (non-usb) to the second x-bee-arduino.
to get started i would suggest the following:
1- wire and get an arduino to talk to your bot, try programming some test movements like left-right, forwards-backwards.
2- if this works, try to control it with your pc: write an arduino sketch that takes commands from the serial port (via usb).
3- finally get that wireless working (if you haven't already), and throw it all together
i think there is a handful of information for the last steps here on the forum or the wiki/playground. if you built the bot, you maybe know how to interface it. there are tutorials on controlling motors and servos though as well.