I do not have the particulars for your model of Boxster, but I can tell you from previous research into buying new rims for my 97 box that you will need spacers on the back of the rims you've specified above. The offsets in the back have to be in the 52mm range, so you'd need at least a 10mm spacer in the back. The front may work, but might rub the plastic wheel wells a little with or without spacers.
So if you're thinking of buying these rims, you're not done spending money if that's the basis of your question!
I do know that there are three kinds of spacers, hubcentric, wheelcentric and one that is a combination of both. From what I've learned here, you want a set of spacers that is both if possible to keep the car from making annoying vibrations at speed. Someone sells them online but I cannot remember who right now... it was mentioned here on this forum about a month or two ago.
Here's a thread that might help you a little with the hubcentric question:
http://986forum.com/forums/showthread.php?t=4595&highlight=hubcentric+spacers
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