The read speed of a class 2 card is going to be more than sufficient for any kind of files you want to put on it. So for things like ROMs, music, video, even to a certain extent applications, class 2 is absolutely fine.
There are situations though where class 2 may be insufficient. It takes a long time to write files to a lower class card, to fill up 32gb you could be looking at hours. So if you change the files on your card quite a lot then it makes sense to go for a higher class card. For example if you want to watch a TV series on your Pandora, then load the next season on after you've watched it, then the next season etc. then you are going to notice the write speed a lot more than if you're just dumping your ROMs on a card once and leaving them there.
Also, if you run an operating system from the SD card you're going to want a good card. Though I/O operation speed is also important here, it's fairly safe to say a class 2 card is going to be awful to run an OS from.
Personally I have a fast card to run the OS and applications/PNDs from. Then a big but slow card to hold my files (ROMs etc) as I don't change the files often.
As the title says today only, this 32GB SD card is £25. The speed isn't amazing but still a good deal otherwise. I'll be getting one as my pandora should be due soon!
Don't put too much store on the class rating - I don't think it is very representative as a general purpose speed benchmark except for when you need to fill the whole card. Biggest risk with these cheap cards is that they might be more likely to expire after a few months use.