From your other post (dual posting not liked...):
Regarding the use of XBee radio modems, I strongly recommend you using the version 1, it is a bout a million times easier to use when you are starting.
Yes, I will second that, they are lower power and with a range of 100m, ample for a remote contol car. The motor shield will do you fine for that but if you are using 4 motors, why do you need a servo motor? I am also doing something similar but using my RC car that has one motor and steering. The steering was centrefugal clutch so I am replacing it with a servo.
What I found is that the motor shiele uses most pins. Therefore you cannot also use the xbee shield. You do however still have all te analog pins so you can use the newsoftserial library to create serial ports on those pins. You want to get the regulated xbee board and then all you have to connect is the serial. If you need any help with code then I should be able to help you with that also.
For the PC end, you want to get the USB xbee board, it makes it really simple to connect the xbee to your pc with a miniUSB cable.
Mowcius