I just wanted to clarify something on class/speed ratings. Some cards use class rating (Class 2, Class 4, etc.), and others use speed ratings (10x, 300x, etc.). Class rating is SUSTAINED speed, and Class 2 = 2 MB. Speed rating is PEAK, and 1x = 150kb per second (like CD Rom speed ratings). So while Class 2 and 13x SHOULD be the same, the 13x is a peak rating, so they're not. Class 2 still kills it.
Also, SanDisk has a habit of underrating their cards. Many people report read speeds and 13MB/sec and Write speeds of 5-6MB/sec, putting these Class 2 cards in the neighborhood of Class 4 or even Class 6. Third parties tend to push the rating, meaning you could be paying for a class 4 that barely qualifies.
Lastly, these higher speed ratings tend to be of lesser quality in terms of reliability, leading to corrupted data requiring a re-format, or a completely useless card. Class 8 and 10 is notorious for this. Class 6 is so-so.
There is nothing you can do on a phone that would require a higher than Class 2 rating today anyway. SanDisk Class 2 is usually equal to or better than a 3rd party Class 4 as well, so consider that when purchasing.
Ok, I'm off my soapbox now