Porsche recommended against using 18" or larger wheels on the 97 Boxster. Porsche upgraded the Boxster in 98 to accommodate larger wheel sizes by strengthening the body structure.
Will you die from mounting 18, 19, or 20's on a 97? Probably not, but the car wasn't designed for them.
http://986forum.com/forums/performance-technical-chat/27209-rim-size-1997-boxster.html