I have a Zoom R8 and the dinky 2GB SD card that came with it has been more than enough for me since I rotate the card in and out often. If you plan on doing a lot of recording over a long period of time then consider getting larger cards such as a 8GB or higher.
Now someone feel free to correct me if I'm wrong, but, I don't think the class speed of the card is going to matter much at class 10. I wouldn't go under a 4 though. Audio isn't as large as video seeing that you are shooting on the 600D I can understand why the thought of a higher-class card came to mind. I only use class 8 or 10 on my Canon 60D and Canon XA10 cameras.