The SD Association recommends the following:
Originally Posted by ford7379trucks
Class 2 : H.264 video recording, MPEG-4, MPEG-2 video recordingPer Motorola, "Android also supports high speed SDcards. High speed means that the bus runs up to 50MHz, which is 25MBytes per second.A rough understanding of the class system, is MBytes per second. So a Class 10 card will write up to 10 MBytes per second. Since the high speed bus can transfer data up to 25MBytes per second, we can support a class up to Class 25 (when those cards are available)."
Class 4: MEPG-2 (HDTV) video recording, digital still camera (DSC) consecutive photo shooting
Class 6: Mega-pixel DSC consecutive photo shooting, professional video camera
Class 10: Full HD video recording, HD still picture consecutive shooting
I had a discussion on the subject of class 10 cards and the Droid Razr with Matt (moderator at the Motorola forums) and he said the Class 10 card will work at rated speed for video recording. But if you were loading the SD card with media from your PC, it would transfer at the Class 6 speed of about 8 MB/s.
I have the Transcend 32GB class 10 ($26.98) on my phone and have no complaints at all with HD recording or playback.