The same question has been troubling my mind recently, so below you'll see the options I've been looking at. They're all in the low to mid 40s (for what appears to be a good one):
Porsche 911 (993)
Porsche 911 (997)
Left field choice (and a possible nightmare): Aston Martin DB7 coupe. It looks soooo good!!
If you don't mind having an automatic:
If staying with BMW:
Or for much less money and more "M feel" an E46 M3. Is it just me or are those looking better and better every day?
The new Cayman should be pretty hot as well, since it should look very similar to the gorgeous new Boxster. More expensive though.