The 8MP CCD in the X does not perform well in low light. Try testing a video in very good lighting conditions. I don't think the H.264 encoder knows or cares what class the sdcard is, it is just going to encode and send it off to the sdcard. If the card can not store the data fast enough it should crash or severely corrupt the video.
Thanks for the info on the sdcards, I was wondering why I takes longer than I'm used to when transferring music, it was even slow using a card reader on the PC.
Mine is C4 btw