ok, if practicality isn't an issue...and your just in it for fun... throw out the Acura. Its a nice car, but your also paying the teh size and weight of a 4-door. Doesn't sound like you need that.
Honda s2000 : where do you live? Unless you live in a decent suburban community, away from the large cities, you never really get to open her up, as there is little low rpm power. More practical for open areas and racetracks than actualy city driving. Also not quite as solid luxory type interior as others.
BMW : i dont like it. Some to. Youll hafta get your own opinion on this one. have you looked at the possibility of a M3?
g35 Coupe : This one gets my vote. Good power, decent handling, infinity luxory build on the inside. Great sportcar, though it will likely be pushing your price cap.
G35 Sedan : If you want something more practical, such as having the ability to take more than one person...id go with this. More versatile than the coupe, but still decent power and a nice ride. Prolly wont hurt insurance as much.
As for the Nissan and Saab....they are pretty large cars. I dont think you would be satisfied with the interior and build quality on the nissan, even though it has power. Its probably a much larger car than you need OR want. The saab is a little smaller and still a nice performer for the money. Saab isn't exactly known for cheap parts when/if it does break though.
My reccomendation...G35coupe. If it gets too pricey, just downgrade a tad to the 350Z
