i'm currently using a SanDisk UltraII class 4 with Max Tranfer speed of 15MB/s 8GB, it's workinging perfectly with my K-x. i also tried some fast shooting and it doesn't give me any problem.
But i notice whenever i used the on camera filter function (such as Fisheye, retro, or others), it needs time to process the photo to save it in the memory card.
But still i haven't tried the video recording function, need to try before giving you any comment about it.