What I should have said is a 986 rear should be in the 44-47 range, so if you want to use a 65, you'll need to use a 20mm spacer to get into that range (65-20=45). Using a 15mm spacer wouldn't get you into that range.
Below is an offset calculator. You can use it to determine how a potential wheel will fit on your Box. To determine what you currently have, go check the rear wheel on your Box at the valve stem. It will have the wheel width (the value with the J) and the offset. This is your starting point. Plug in the new wheel dimmenstions and the calculator will tell you how far in or out the new wheel will be. You can then measure and see how well that works.
http://www.1010tires.com/WheelOffsetCalculator.asp
Do two things before you purchase any wheel: (1) check the offset calculator and (2) search the forum for information - there's tons of it here. Once again, if you search on et65, you'll find many threads about these wheels. You're not the first to ask. 20mm spacers will work but there are compromises that have to be factored into the decision. Some are willing to accept these compromises, some aren't.
DON'T buy anything until you're sure it's right for you. Porsche wheels are expensive and no one likes to make an expensive mistake. Many sellers say their wheels will work on a Boxster, but only if you use very thick spaces. Be a wise consumer.