A few things to consider when implementing car physics:

Different turn rates for inside/outside wheels:
http://en.wikipedia.org/wiki/Ackermann_steering_geometry

Anti-roll bars to prevent the car from flipping in cornering turns:
http://auto.howstuffworks.com/question432.htm

I don't know, Marco. The anti-roll bars in my old Vauxhall didn't do much good.

Heheh. Marco's post should read "Anti-roll bars to HELP prevent the car from flipping..."

A decent method is to:

1) limit the steering as you gain speed

2) apply a downward force relative to the vehicles orientation upon turning.

A roll bar may work, but it depends on the implementation. If it's invisible and sticks out very far it'll catch things and toss you around. In this situation it's best to take a cylinder entity, apply it to the vehicle via a looser hinge joint, and have it set to the same collision group of the vehicle(passable unto each other). So long as the hing joint is loose enough you shouldn't catch very often and will have a nice "rollbar" radius...