I tested a class 2 card on my old H2 and it worked fine at 24/96. You should be fine.
The only times I've had card speed problems are in my camera, shooting 720HD video. Class 4 is not sufficient if there's a lot of action. But indoors, without much panning or zooming, picture compression can let you shoot some good stuff on class 4. You just risk cutting it off if you get too nutty. Class 6 is adequate for HD720, in my experience. I would assume that class 6 or 10 would be necessary for 1080HD.