I know about the difference between Class 2, 4, 6, and 10. But is there any difference between them in terms of loading apps, playing/recording video, system responsiveness, etc?

The class usually refers to read/write speed of the card, so yes the card can have an affect on terms of loading information depending on its speed. I would assume that a class 4 card would be good enough for reading music.

Given the low cost of memory these days I'd go with a good class 10 card.

I've got a Patriot 32GB cost under $40 and a 16GB is under $20.

I have an x710BT and I loaded up some mp3 files on my 16 gig Class 10 card. It recognizes and begins playing, however, as soon as music starts to play, the whole unit freezes up, and i cannot use the touchscreen, menu/map buttons. Does anyone know if this would be because of the memory card size? in the specs, it does mention unit maxes out at 8gig cards. any insight?