Any 7D users out there that can comment on if a 7D can gain any benefit from the 1000x vs 400x. I've got a couple 400x and occasionally will max the buffer. Always figured it was the camera, but with cards on sale, might as well ask if it might be the cards! If a card would also benefit me, could have quite the boost once the new firmware is released!
I'm not the expert on this. (paging Dr. Neuro), but from what I understand, there are really two issues. How quickly the camera writes to the card and how quickly the card transfers the data to the computer. The speed that the camera writes to the card is limited by the write speed and buffer of the camera. My understanding is that most cards can accept the data faster than the camera writes from the buffer to the card, so there isn't any advantage to the super fast cards for the 7D. (400X should be more than adequate).
On the other hand, if you are using a card reader to transfer the data to your computer, it will go a lot faster if you have a fast card.
My real life experience tends to confirm this. I've never had a problem with the 400X cards (although I wouldn't go for the slower ones).
I make no claims to be a "techie" so I've probably screwed this up, but I think the bottom line is that the super fast cards don't offer much if any advantage because the camera becomes the main limiting factor.
Anyone who knows more about this, feel free to correct.