There's a good chart of cards here: http://elinux.org/RPi_SD_cards
. Scrolling way down that page you'll see some performance results. It looks like the RPi tops out around 20 MB/s speed reading from an SD card, so honestly, in my opinion, anything over Class 6 is a bit of a waste. Mind you, these days there's not a lot of spread in prices so a basic Class 10 should be a safe purchase, as long as it's not more expensive than the same size in a class six. BTW, all the words like "Ultra', 'Extreme', 'Professional' are just marketing BS. The Class determines the speed, and anything above 4Gb has to be SDHC and anything above 32Gb has to be SDXC. SanDisk is the largest manufacturer of SD Cards, so they are a target of counterfeiters - don't buy cheap SanDisk cards from sketchy looking deals on eBay. Lexar is a good brand, recommended by some camera manufacturers, and I've never had a problem with a Lexar SD Card.