The standard Cayman engine is the 2.7 liter flat 6 that makes 245 horsepower. The more powerful Cayman S gets a 3.4 liter flat 6 cylinder making 295 horsepower. The Cayman S was actually sold first before the Cayman, beating the latter to market by several months. A Cayman RS is likely in the future, and it will likely have a turbocharged version of the flat 6 engine and have tuned features for motorsport racing.
The Cayman S is already a hit on the race track. It has been tested to be equal to and sometimes better than the flagship Porsche Carrera 911 in terms of performance. That's not good news for the Carrera 911, as its base model costs about $12,000 more than the top level Cayman S.
The only thing the Cayman S is missing that some owners would like to see is a limited slip differential, which the Carrera 911 does have. It has been speculated that the Cayman does not get this feature because Porsche does not want the Cayman to take too many sales away from the 911.
The Cayman retails for about $49,500, with the Cayman S selling for about $59,000. You can find a few of them on the used market already. There were several listings on eBay the last time I checked.