Just to give everyone a frame of reference, let me explain the class terminology and give some examples:
The classes refer to haw fast a card can be WRITE. This means the phone can WRITE data to the phone at that speed. Writing to the card is usually slower than reading, but that speed is not formally required to be any specific number. Supposedly Brand names are faster than generics, but that is up for debate.
Class 2: data writes at 2 MBps (Megabytes per second).
Class 4: Writing at 4 MBps
Class 6: Writing at 6 MBps
And so on.
How does this affect us, the users?
Watching movies:
iTunes (or Windows Media) Digital Copy movies from purchased DVDs (or download) play that the rate of 1.7 MBps. This means the video will play at full speed with no freezes or skips on a class 2 or faster card. (If the phone processor can process the data fast enough, which I expect it can).
iTunes HD video (720P, which EVO can handle) is 4.5 MBps, which means a class 2 card will stall out, a Class 4 MIGHT play it fine, but a class 6 is your best bet.
Home Brew Video: I encode H.264 video using XVID4PSP v.5 using the following rates; DVD => 756 KBps (0.7 MBps) HD 720P => 1,780 KBps (1.7 MBps). (Fast action movies I bump up to 2.5 MBps - the faster the motion in a frame of video, the harder it is to encode with detail.) These are MUCH lower than the Digital copies sold, but since I encode them myself, I set the encoder to Super-high quality and let it run, knowing it will look just as good as the purchased versions in MUCH smaller files, they just take longer to make. These will play fine on any card rated Class 2 or higher, even the HD files.
RECORDING Video:
The EVO can record video at 720p (1280X720) at a rate of 20 frames per second. This is slow and choppy, considering 24 frames is movie/film speed and considered the slowest acceptable. I'm betting the reason is saving video in h.264 format at real time speed is very hard on computers. Converting a 1280X720 movie that is 2 hours long takes 6 hours on my video editing computer (3.5 hours on an 8-core Core-2 Rendering machine with 8 GB RAM), so realize the video recording is not going to be NEAR professional quality. HOWEVER, I think I remember reading that the EVO saves the 720p video at around 2 MBps, so the provided 8 GB class 2 card will work.
So, for running apps, listening to audio and playing DVD quality videos, a Class 2 card will work.
For watching home brew HD video also, get a Class 4.
For the best quality and performance on iTunes-level videos, get a class 6.
Hope that helped a bit.