A Class 2 SD card is supposed to be guaranteed to read and write at a speed at least as high as 16 Mbps. The highest quality MP3 files you can get from Google are about 1/50 that size. A completely uncompressed CD rip has a data rate of about 1/12 that much. Finally, a full, high resolution, uncompressed DVD-audio rip playing at 24bit/96hz has a data rate that is about 1/4 of that speed.
Basically, even the slowest Class 2 card has more than enough bandwidth to play any type of audio stream you want. A faster class will really only show up when you are copying audio files to the card.
Get a quality brand name though, as not all vendors are legitimate in the speed ratings. Sandisc, PNY, Amazon, Trancend are all good names.
-Suntan