1. Large capacity cards have a big disadvantage in that at about 24-26MB for a 24MP sensor, there are about 40 images per GB or more than 2,500 per card. That is a lot of images to scroll through and down load. The only reason to have these cards is to store video. I have experienced slow down with 32GB card, especially while downloading images to the computer. For still photography 8 GB is an optimum size with the 24MP sensor. That will accommodate more than 300 images. If you want more capacity get multiple cards, that is an insurance against corruption of a single large card.
2. The size of a 24MP 14 bit raw is 24*14/8 = 42MB, so if you shoot 5 frames per second you need 5*42=210MB/s transfer rate. Neither the camera nor the computer can sustain this, so very high speed cards may be an over kill for most cameras. Now for medium format 80MP sensor the image size is 80*2=160MB, and even at 1 frame/sec a very high speed is desirable. The highest sustained rate in D7100 for jpeg (which are around 8 MB max). So 7*8=56 MP/sec max transfer speed required for the card. Higher speed card is good to have, but if you are paying through your nose for speed then it is not worth it. For 24MP sensors a maximum of 60MB/sec is all that is required. For D3xxx series with 12 bit RAW, the transfer rate is below 45MB/sec.
