You do not need a class 4 or 6 to watch movies or music.
Really think about this. At 2 MB/s a 600 MB file will take how long to write?
What 5 mins? If you read the thread on the subject it is a lot quicker, more like 1 min and 40 some secs. But lets say 5 mins.
Now read speed is about 3 times quicker then write, but lets say that it is the same. You would have to watch a 2 hour movie in less then 5 mins, to reach the minimal speed of the card.
If you are watching a 2 hour movie in 5 mins. There is something really wrong.
A class 2 card will not slow down you video or your picture taking either.
In fact with the htc evo. Class 2 is all you really need.
The video is 5Mb/s. Class 2 card is 16Mb/s, 3 times the amount needed. Pictures are limited by internal buffer, not the card. Music and video will not be effect. The only reason you should get a class4 or 6 card is for apps, which probably will not see speed improvement.
Bottom line it is your money. Spend it as you like.