I've been reading through this thread and it's full of useful info.
However for the class of Micro SDHC card required for a tablet a class 4 card should be fast enough for whatever you're doing.
( Disclaimer: I'm not an expert but I've done my research so I would regard myself as a well informed idiot.
The only things that the class of memory card would have a real effect on would be time it takes to transfer files onto the card and the boot up times of games. It shouldn't have any effect on SD or HD video playback.
The sort of HD movie file you would be playing back on a tablet wouldn't be like the 20-odd gb size of file you'd get on a blu-ray for an average movie. You'd instead be playing a compressed HD movie file of around only 4-6 gb for an average movie which translates to a file that is around only 35-50mb per minute of the movie which is less than a mb per second of the movie. As most people use SD files (especially on a 7" screen where HD won't make that much of a difference) you can see that having a class 4 card isn't going to bother you in the slightest. This is because a class 4 Micro SDHC card should be reading at a minimum of 4MB per second which is more than fast enough to play back HD videos let alone SD videos.
If you are watching an HD video file and it's stuttering then it's more likely to be apps running in the background affecting your playback and all you have to do is turn them off to fix the problem.
So as I said before the only real advantage of getting a higher class of card would be that it would speed up the time it takes when transferring files to the card and it would shave a few seconds off the time it takes to boot up games or load new levels in games.
The higher class memory cards are really aimed for people using HD video recorders as the write speed required to do that has to be much, much higher (they use about 11gb per hour of footage) compared to a compressed HD video file (about 2-4gb per hour of the movie) playing back on a tablet.
After saying all that however if you can get a higher class of card for not much more money then there's certainly no harm in doing so.
If you're on a budget though a class 4 card should suffice and the only disadvantage you'll have is when you're twiddling your thumbs waiting when transferring files on to it or having to wait a few extra seconds for games to load up.