Great advise so far!
I love 993 Porsches, those being the 1995-1998 air-cooled cars, but you said no used cars, so...
Are you looking for any particular quality? If you're looking for great materials and high build quality, you'll be able to narrow this list down a bit. If you're looking for a weekend trackable car, the same can be said. If you're looking for all out sportiness, only a few really stand out. They're all great cars, you really have a nice list going. If you want a comporomise of lots of nice things, an M3 is a great deal, if you want a fun track car, the Boxster S is superb, and the Z06 is really in another performance category from all of the others, we won't talk about the interior!
More than anything, try to figure out a general group of qualities that you value and narrow this list down and then drive all of the cars left. If possible, find a lightly used model to drive, that way the engine and brakes will be broken in and you'll get a more accurate feel for rattles, etc. Of course, the RX-8 and S4 won't be available to test, at least yet. Good luck with your search!
Ben