For still shots, SD card class is almost not significant unless running burst mode. RAW files from my 15MP 50D run between 15 and 27MB each... Even a class 10 SD card (the 50D uses faster CF cards*) will take 2-3 seconds per image so you become limited by the internal camera buffer (the camera may take 4 frames per second, but will pause when the internal buffer fills, which may be as few as 10 shots. (Medium -- 8MP -- JPEG is running 2-4MB each... or 2-3 per second for SD card transfer).. Video is only transferring ~2MP, and compressed more heavily than JPEG, so under 0.5MB per frame.
Note that, per the link, Class 10 is for an unfragmented card... If you've shot a few images, then delete some, then start a video, you may not achieve the 10MB/s rate. Class 6 and under tend to be for cards that may be fragmented.
* My near decade old SanDisk Extreme III Compact Flash cards are 20MB/s in the 2GB size, and are the SLOW CF cards I have. My 4 and 8GB CF cards are minimum of 30MB/s -- my 50D can even use UDMA CF cards which can exceed 45MB/s. And none of my SLRs do video -- though the 50D does do ~6.3fps and JPEG only with UDMA can buffer up to 90 shots (or nearly 15 seconds of burst mode... RAW drops it down to only about 10 [R+JPEG] or 16 [R only])