Only advice I've seen is to stick with a brand name such as Sandisk. Also the only importance the card class has is if you are heavily into video recording.
Please be aware both of the previous sentences are based on stuff I've read on these boards. I do not do video recording and I do not have a faster than stock card. Just passing along what I've read - (it was on the internet, so it must be true!! Right?) :icon_ banana:
I have a post in the Droid X forum (moved there for some reason). I have a Sandisk 8GB card in my Droid Pro as does my wife in her Droid X. Both phones repeatedly gave us the "SD Card is Blank or has Unsupported Filesystem" and after the last time both cards won't format and aren't recognized by my laptop.