Hi

I agree with the idea of more cards of smaller storage capacity in case of loss. I currently use 8Gb & 4Gb cards and carry a bunch in my kit(Canon, sorry).
As for the Mb/s, you are correct, in that is the transfer rate between camera and card that the card can handle. So think about if you are using burst mode, especially high speed burst, then you may see the camera buffer filling up if the card is only a slow transfer rate. I use 60Mb/s cards, as I generally do not use the 8 frames a second burst mode, only 3, which means I'm looking at a total of around 60Mb per second transfer rate(3 x 21Mb per RAW file) needed, so that's what I get.
Hope this makes sense,

Regards Chris