Great question I got the other day.
What is the difference between Memory Card Speeds represented by "X" Speed, vs Memory Card represented by "MB" Speed?
I will try to keep this simple:
"X" Speed is the equivalent of the maximum CD audio data transfer rate.
A Memory card that is 100x for example, has a transfer rate roughly 100 times faster than CD audio transfer.
This CD Data transfer speed is roughly 150 kbytes/s, so the conversion equation in terms of Data Rate is=
Rate = (Speed in "x") x 150 kbytes
In the case of this Delkin card;
633x x 150 kbytes = 94,900 kbytes (divide this by 1000 to get MBs) = ~ 95 MB/s (which is a pretty dang quick card for a camera)
A card with a 133x rating is about 20 MB/s
- Memory Card Speeds become more and more important when recording or capturing large file sizes. If you are shooting small jpegs, its not going to matter as much as shooting HD video on your 5Dii. If you are capturing HD Footage, UDMA are the ones being recommended now, with a speed of at least 45 MB/s or higher.
That said, I know of many photographers who claim to shoot video on 133x without many issues.
My stance on it is this: Get the biggest capacity and fastest speed you can if you are shooting video.
If you shoot RAW images, 133x will be fine.
I will also say that if you look around, you can often find some great deals on fast cards from different manufacturers.
Here are some AWESOME performing cards for recording video on Bhphoto.com
(Yes, they are expensive)
Extreme 32 GB- 90Mbs
Extreme 64 GB- 90Mb/s
Extreme 128 GB- 100MB/s
3 Individual 32 GB- 600x