Personally, I would keep the 996 C4S and wait for the Turbo.
But if the Turbo is not a real option (too much money, too aggressive look, etc.), go for the 997 C4S Powerkit (X51), this would be a good choice.

The 997 Turbo will be the hell of a car, so maybe you want to wait, even if it is going to be tough.
The 997 Turbo is expected around late summer in the US, a "broader availability" of cars should happen around mid 2007 (it is pretty difficult to get a 997 Turbo in the first couple of months after official introduction).