Just found this information here
Best Memory Cards for Android Phones | Smart Mobile Phone Solutions .
I'm going to try a class 10.
"Choose one with class
When choosing a memory card it is very important that you consider its Class. When I say Class I?m not talking about the adjective meaning stylish excellence I?m talking about speed. Micro SD cards use Class to determine transfer speeds. The Higher the Class the faster the phone will be able to read and write information to and from that SD card.
If you buy a memory card with a Speed Class of 2 (lowest Class) then your phone, camera, or device can only write 2 MB/sec (two megabytes per second). A Class 4 SD card can only write 4 MB/sec, Class 6 is 6 MB/sec, and Class 10 will write 10 megabytes per second.
Some devices won?t even work right if the memory cards Class isn?t high enough. For example if you have a camcorder or video recorder built for a class 6 memory card and try to use a Class 2 or Class 4 SD card then your videos might error, might be fragmented, read ?busy?, or might just error out and not work at all.
So when shopping for an SD card if your given the choice between multiple memory cards of the same size then don?t simply choose the cheapest option because chances are it has a lower speed class rating and is not necessarily the better product. You don?t want to have to wait for your pictures to open in your gallery, or wait 5 minutes to transfer photos or videos from your phone to your memory card, and if your camera is HD then you?re going to want to make sure your videos record and save properly to your SD card."