Class 2 is fine for stills. I used a class 2 for a video, and it started to ditch frames after a couple of minutes in an attempt to keep up. I have class 6 cards, and they're fine - which is probably why they were recommended.
One other advantage of higher speed cards is how quickly you can get images off them again - assuming your card reader is a fast one, and they do vary wildly. Alas, the class doesn't matter so much here. When it comes to reading, my SanDisk class 2 is faster than my "play.com" class 6. It's also faster at writing, certainly for small files. The class only guarantees a minimum write speed, which is what matters when recording video. It's a guarantee that there won't be a buffer overrun, where incoming frames arrive in the buffer faster than they can be saved.
My fastest card is a Kingston 60x 2GB SD card. It out-performs my SDHC cards by nearly double, reading and writing, although I don't know whether it can sustain its write speeds at a minimum rate. I was testing average speeds over a gigabyte. During that time, the write rate could have fallen to a rate that was below that needed for HD video.
