I also just got a set of the 18" Carrera Wheels like randelneighbor for my 2000 2.7L Boxster. I got the cheaper Falken 512 tires on the wheels. The folks at Wheel Dynamics were very helpful and answered all of my neophite questions.
I understand that because there iare no offset issues with these wheels all I have to do it take off my 17" wheels and bolt on the 18" wheels and that I do not have to reallign my car, is that the case, thanks,
Ed