Ignore the class rating. Higher classes (espcially class 6) on microSDHC tend to be less reliable, leading to corrupt transfers, or shortenend life. Also, Class 6 (or higher) is generally something you need for high end cameras or HD camcorders due to the transfer rate.

For media playback, Class 2 is more than adequate. When transferring files, if you're using a card reader, you'll notice a difference between class 2 and class 6.

In summary, avoid the class 6 cards. They're overpriced, less stable, and are intended for professional recording. You'll get no benefit from using one over a class 2 in a PMP.

Source: http://icanhaz.com/speeds