Not sure what your budget is but you may want to consider a stand alone recorder solution, preferrably with solid state storage. A race car will have a lot more vibration and shock than a regular car and a hard drive may not be able to take it. Also getting 2 to 4 cameras at 30FPS with decent res is going to take a fairly powerful computer and capture device(s).
Ovation System's Flashback2 would be ideal but it is pretty pricey.
Guys I know in racing have used SSD camcorders on mounts with good success, but finding one with an external video in (from a "bumper cam" board cam for instance) will probably be tough.