ABS Upgrade

I have a 2004 Classic (bare bones, but I like it) and was wondering how expensive and difficult it would be to install an ABS system in the car. Does the ABS have its own computer or is it part of the existing computer? The front hubs already have the sensors, what about the rear? Or would it be better just to trade it in for a real Malibu?

Why the burning desire for ABS brakes? They prevent the wheels from locking up, but I would not try and add ABS to a car without the system. ABS has a separate module with a computer and I belive that it is tied into the PCM.