First, if you haven't bought the TReX yet, you should consider the Simple Motor Controllers. Chaining two of them together can be as good as the TReX, if you do not need the TReX's AUX motor port. Plus, the simple motor controllers have USB and a really nice configuration/control GUI.
For your wireless connection, you need to decide what kind of connection you want to use. Two common choices are Bluetooth and WiFi. I haven't used either of these boards, but they are worth looking into:
Another option is to go with one of the 2.4 GHz solutions like XBee. If you go this route, you will have to buy two modules, one for the computer and one for your robot.