The 2003 Mercedes SLK320, used 205/55/16's on the front, and 225/50/16's on the rear (same diameter as the Crossfires rear tire). They will fit right on the Crossfire, but it will look a little anemic with them. They will improve the ride, and (if your car's an NA automatic) they will make your car about 3/10's faster in the 1/4 mile, maybe better. But your car won't handle as good on a twisty road.
By the way, welcome to the forum.