I bought a 16GB SanDisk class 4 on Amazon for $26. If you're going to transfer a lot of HD video back and forth from the SD card, then I suggest something higher in class, but for everything else class 4 is A-OK.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?