I have the nav system in one of my '99 Boxsters and in my '99 996C4. The problem is, it takes CDs to load maps and Porsche isn't supporting the upgrades. There are (I think, if I remember w/o looking) three different components to the system. It fills three of the four slots, with the heat/AC unit filling the fourth.
They were a terribly expensive option ($3K+) when new, but unfortunately, it's now totally worthless.
I've read that as part of Porsche's "Classic Program", Porsche is bringing out an updated unit for the 986/996 models. I've done a little looking around, but haven't found the new system yet.
But for now, save your money.