Not All SD Card Speeds are as They are Labelled

Technically, a class 2 SD card is the slowest, and a class 10 the fastest.  But this is not necessary true in real life.  There is a useful chart in the CyanogenMod Wiki that shows how you can run a test.

In the test results so far, we can see that a class 2 SD card by Sandisk is actually faster than a Kingston class 4 card!

What does this mean for you?

I suggest that you stick to a name brands, and don't get a cheap no-name brand from ebay or something.  Buy from a quality store that accepts returns like Amazon.com or Newegg.com.  Then if your card is not as advertised, at least you can return it.

Also, read the reviews on the cards, people will often write about the performance the experienced on the card.

I personally don't think you need anything more than a class 4.  I have a 16GB class 4 Kingston card on my EVO and I don't experience any slowness when playing videos are viewing pics.