I would have to say that if you for some reason really can't buy a wheel, either because of the cost, because of the space it takes on your desk, because your parents/girlfriend/wife/boyfriend/husband would kill you or some other reason you might want to consider a gamepad with analog sticks. It won't give you the realism of a wheel, but it will give you the possibility of driving with basically the same controls as for RC cars.
Obvious advantages over mouse driving:
* Throttle/brake separated from steering, with both still being analog
* Steering returns to center if you let go
* Plenty of well buttons within reach
(The controls are of course less intuitive and more sensitive than a wheel, so really, if you can get a wheel, do so.)
An example of a good gamepad that would work is the Logitech RumblePad II, but I'm sure you can get away just fine with something cheaper.