Here's something a little more technical: Motorizing 8448 supercar. I am using Power Function parts from my monster Dino to do this motorisation. Below are some pictures.
The 8448 supercar with gull-wing setup. This design allows a HUGE space in the boot for motorizing it.

The steering mechanism. Originally wanted to have it set up so that the steering wheel can turn also, but not enough parts (need those universal joints connecting the steering wheel) to do it for front wheel drive.

The XL motor connected to the engine. As you can see it is well connected to the gearbox and chassis. This is crucial since the XL motor is quite strong and you need a high torque to turn the wheels.

Another view of the steering and XL motor with IR receiver. The white driver's seat was taken out to do this shot. Unfortunately I am not able to fit in the passenger side seat.

Battery compartment.. well, I just stash it in the boot

Easier to replace and connect/reconnect.

Another shot with remote. As you can see, the remote is SMALL compared to the whole car.

Overall you can't really go fast in this set-up. This is due to the gear setup as well as the friction that the motor has to overcome throughout the gearing. The best gear to drive is 1st gear. Using any gear higher the car will move very slowly (like a real car, trying moving from standstill in 1st gear). When the car is moving the V8 turns too!
@ weiwei2, the jeep is not Lego, it is a clone brick jeep. More here
http://www.brickshelf.com/cgi-bin/gallery.cgi?i=2668218 
I know coz I have 1 too, but not built yet.