Read about it here:
http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101
And calculate it here:
http://www.1010tires.com/WheelOffsetCalculator.asp
I don't think those wheels will fit your Boxster though because the rear width (11in) is too wide. Max That will work, I think is 10.5, and that is pushing it.
I'm no offset expert, so others should chime in, but the original offset offered on the 17in Boxster wheels was ET 55 front, ET 50 rear, so depending on what the specs are on your new wheels, you may need spacers. Don't forget, with spacers, you'd then also probably need to buy longer wheel bolts.
I found it easier just to find a set of wheels that have an offset and width made specifically for the Boxster so that I didn't have to add spacers at all, and could just bolt them on without having to buy longer bolts either. The wheels I bought have the following specs:
Fronts 18x8 / ET49
Rears 18x9.5 / ET43
