If you're looking for the fastest SD card out there, make sure there's an "I" symbol on there as the SD Association has redefined the code for speed.
First, the SD Association has introduced the Ultra High Speed (UHS-I) classification for SDXC and SDHC cards. With that "I" marker, that means these hi-speed cards are capable of transferring data at a speed of up to 104 Megabytes per second. The UHS Class 1 symbol also signifies that the devices supports real-time, HD video recording.
Pretty nifty, but these symbols might be hard to notice, so do pay attention when scouting them in the future. Memory cards and products that support them should start having this branding soon.