The more expensive cards have second interface chip which helps with the speed.
Sorry rhanesworth but the Rob Galbraith test is a joke trying to assess data transfer rates by standing over a camera with a stop watch and timing by when the write light comes on & goes off - well try presenting that as evidence to any engineer. If it were possible to accurately measure the data transfer the manufacturers would have done it, but it's not that simple.
memory cards have a minimal effect on the frame rate, and the maximum number of shots unless they're very slow, now you need a resonably quick card for video on a DSLR, but the single most important thing is reliability. A whole days shooting can be worth over Â£1000 it's not nice having to go back & do it all again simply because a cheap card has failed!