If you notice no issues, I would button it back up and then buy a factory OEM serpentine belt, to do at a later date, instead of using the Goodyear belt. One Boxster owner on another board actually kept his serpentine belt on for about 100k miles before his belt crapped out on him, but it was an OEM belt.
At $28 dollars for the OEM part, I'd go OEM:
http://e-partssales.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=P&Product_Code=99610215166&Category_Code=ENGMECH
I changed mine at 48k miles as a preventative thing, but my old belt was still in good shape, so I kept it in my front trunk as a spare in case something happened to my replacement for any reason.
Here are the DIY instructions on how to replace the polyrib belt:
http://www.realtime.net/%7Erentner/Blog/B600322026/C1691662107/E397927809/index.html