Following the wonderful example of Randall, and seeing in another forum that a dealer wanted $150 for a battery and another $150 to install it and "reprogram" everything (put in the radio code), I thought I'd pop out a few things to hopefully save someone some $$$.
If you go to the auto shops, many times they will not have a battery listed for the Boxster so people end up at the dealer or think it's special and must be a Porsche battery. The truth is there is no difference between a Porsche battery and any other. You don't need a model number at the store to buy the right battery. All you need is the minimum cold cranking amps and the right size battery to fit properly. You can either measure or take your old one with you.
Edit: I forgot to mention the ventilation tube. You need to buy a battery with the connection for the ventilation tube that is used in the Boxster.
Here's why many times it really doesn't matter what brand you choose:
Battery "brands" are pretty funny. There are only three companies that make automotive batteries for sale in the US. My wife used to work for Exide and would laugh all the time because, being a woman, every time she told a man she worked for Exide they would automatically tell her what they thought was the "best" brand battery. Most of the time it was an Exide battery with another label on it. The only difference is the quality specs the vendor required. Take your pick, Everstart (Wal-Mart), NAPA, Auto Zone, Champion and many others are all Exide, but the Champion and Everstart (Wal-Mart) batteries are actually built to the highest specs because the vendor demands it. The others are the same, with different colored labels and prices.
A lot like American cars, eh?