I've been thinking about this all day (I know, scary isn't it). With a ceiling of $40,000 and change, there really aren't too many inspiring cars out there, particularly if you want something you can comfortably drive daily. If you were looking for a toy, there are many offerings in that range (the 550 Spyder and Cobra replicas immediately come to mind). I have to say that if you don't like the 996 and you don't want to stay with the 986, the NSX is not a bad choice. Still, you're going to be pressed to find a good, later model example in the price range you're talking about.
The Mercedes SL500 and SLKs really don't do it for me, especially after my experience with my last E Class. It probably varies by area of the Country, but around here, everybody seems to have one. They're like the Annapolitan Corolla or something.
To me, the MGF is still a pipe dream and even if it does happen and appears on our shores, it looks eerily like this...
I guess if it were me, I'd upgrade the Boxster or step up to a 987S ...which is probably why I bought a Boxster when looking for a fun rag top in the first place.
Good luck.