Contrary to what you may have been reading, you don’t need a fast memory card to record video. Actually, most cards manufactured in the last few years should be fast enough since the data rate required by H.264 video is much smaller than what is actually used when taking pictures – as long as it is greater than 30MB/s (see below) – note that is usually also seen as 133x – so anything greater than 133x speed will be fine.

There have been some comments that you can get some artifacting in your video if you use slower cards, and I asked Canon about this when the Canon EOS 5D Mark III was announced:

is there a ‘minimum’ card speed recommended for the Canon EOS 5D Mark III to handle the All-I compression?
CW: We recommend using memory cards with write speeds faster than 30 MB (not Mb) per second for the ALL-I setting. The faster, the better.

What about the buffer warning icon then?

This icon appears when the camera buffer is filling up too fast, meaning it can’t keep up processing the footage (ex: too many details to encode), not that it can’t copy it to the card fast enough.

Still, fast card are good because…

The benefit of faster cards appears when it is time to transfer the footage to the computer. A fast card (plugged in a fast card reader) can empty the card up to 5-6 times faster which can be a real time saver when you have to empty 4 of these at the end of the day.

Additional info

B&H Photo/Video has an in depth article on memory cards

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planetMitch

chief astronomer at planet5D LLC
Mitch "planetMitch" Aunger is the creator and mastermind behind planet5D.com — which covers every aspect of HDSLR cameras (and a whole lot of other cameras as well). A lover of photography since his early days, he graduated to selling stock photos to make a little bit of spare cash. It wasn’t much, but it allowed him to upgrade to better cameras as well as computers, and it wasn’t taking money away from feeding his family.
He loves the stories that the still image can tell. In the summer of 2008, he read a story written by Arnold Kim called “I Quit My Job” — Arn is the owner of macrumors.com. He started macrumors in 2000 as a hobby, and by 2008 he was making more money with the site than he was on his day job, so he quit. The surprise to the story is that he was a doctor making more than six figures! Right then and there, planetMitch dreamed about doing the same thing.
September 2008 rolled around, and Canon announced this new version of the incredible 5D — the Canon EOS 5D Mark II. It was the first Canon DSLR to shoot full HD video and the earth moved! (It really was a huge deal in the photography world.) planetMitch knew if there was ever a chance to ride a wave of something he was really passionate about, this was it — and the blog at planet5D.com was born. It is now one of the most popular HDSLR blogs on the planet, and he is making a full-time living from blogging. He couldn’t be happier!
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