The wheel you have is called the Boxster twist and was the 17" option over the base 16" wheels although most Boxsters delivered in N America came with the larger 17 that you have. A 1/2" wider version of this wheel was the base wheel for the 996, but it too is fairly rare as most 996s were ordered with the optional 18" Turbo/Technology wheels.
As Blue2000s mentioned the easiest way to achieve the look you want is to add spacers, and depending on the type of spacers used longer wheel bolts.
One thing to keep in mind is IF you plan on plus sizing your wheels to a larger diameter is that Porsche does not recommend using wheels larger than 17" for the '97 model year. The suspension/body mount locations were strengthened on the later years to accommodate 18" wheels. With that said I've not heard of any problems with using 18" on 1997 models used on the street.
Wheel Enhancement is a company that specializes in Porsche wheels and has a good online catalogue of all the various wheels used on Boxsters, and their showroom is in Culver City if you're in the area.
http://www.wheelenhancement.com/index.php?t=WheelDetail&id=35
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