If you need a battery, you might want to consider an AGM (Absorbed Glass Mat) battery. This type of battery can be of particular benefit in a Boxster because it does not need a vent, and therefore will not leak acid nor will it need added water. I have wanted to try the Deka AGM group 91 battery in my car, but as it turns out my battery passes load testing just fine. Here's a link:
http://www.remybattery.com/Products/AGM-European-Performance-Battery__9AGM48.aspx
It's 2x the price of a conventional battery, but it should be much longer lasting. Unlike the similar Optima battery (also AGM design), the Deka is a direct fit into the Boxster with no modifications necessary.