I vote S2000 as well, especially since you already have an E9X M3. They would make a great pair and you can't beat the price. I'd have one now if I had room for it.
Cayman S (had one for a couple years) or Z4M would also be a lot of fun.
I've never really like the 996 turbos. The turbo pull is great, but other than that they feel a bit heavy for a "sports car".