First off, we need to clarify. Are you looking for a car racing
game, or a car racing
simulator? As you know from you flight simming, the former has a primary focus on graphics, fun, and eye candy. The latter has a focus on physics, setup, race strategy, driving style, etc. Both have their place.
Serious racing sims include rFactor, GTL, GTR2, RACE, Live For Speed, Richard Burns Rally, and Grand Prix Legends. Racing games are dominated by the Need for Speed Series, but also include Colin McRae: Dirt and others. Test Drive Unlimited is sort of halfway between the two genres.
IMO rFactor hands down has the best triplehead support in a racing sim. The HUD does not stretch, you have control of eyepoint position in-car, you have control of the FOV at a mouse click, and best of all, it supports a feature called "multiview" (which you can read about in
this thread). GTL works pretty well in triplehead, GTR2 has some issues online at higher triplehead resolutions but there are reports it works ok at 2400x600, and I don't know about RACE. All of the above games are based on the Image Space Incorporated gMotor2 engine.
Live for Speed I don't know about in TripleHead, Richard Burns Rally looks great in the screenshots and videos that I saw but I was never able to get it working right, and Grand Prix Legends works quite well with a few quirks and is very impressive in triplehead.
I don't have many racing games, but I do have Need For Speed: Most Wanted and it is very good fun, especially the police chases. I run it in single-screen, though, because the stretched HUD really bugs me; my driving in the game relies heavily on the overlay map, which doesn't do too well when all stretched out. I don't have any other driving games, but I believe Test Drive Unlimited works pretty well in triplehead.