What speed of card is best to play movies and what is the best format to put them into? I'm thinking of getting one of these tablets to play movies and want to make sure it can play movies without freezing.
The faster the better. You can get a class 10, 32 gig for around £20, that's the best bet (also the largest card that the tab can use).
As for files type, while it can play .mkv (HD 720p and up) the file size of HD movies would mean that you'd only get 2 or 3 on a 32 gig card, and on a 7" screen HD seems a bit pointless. And full HD would be pushing the cpu/gpu to the limit. If you stick to .avi's a full movies (90+ minutes) is only around 800 meg. So you can get lots on the sd card.
Some confusion here. .mkv and .avi are just different types of containers for packaging any number of various types of compressed video and audio. Most types of compression can be used with either of these container formats. So whether a file is an avi or or and mkv says nothing in itself about how the video and audio was compressed, and nothing about its size or quality or the demands it makes on the hardware.
I personally have many .mkv files with low definition material and many avi files with HD content.
For technical reasons mkv is preferred over avi by people who have to process video rather than just watch it and it does have some minor advantages for end users (e.g. the container can hold multiple sets of subtitles in plain text file format). It is also sometimes wrongly claimed that .mkv files are significantly smaller than .avi versions of the same quality, but that's another misunderstanding. It so happens that .mkv files often use H264 video compression, which does indeed produce smaller files at any given quality level than older forms of compression, but then avi containers can hold H264 video just as well, and .mkv's can also be created with compression less efficient than H264.
A video using h264 compression at 720p, no matter whether its container is an avi, an mkv or an mp4, comes out at around 2.5Gb for a 2-hour movie. So a decent number would fit on a 32Gb SD card even that definition. If you settle for compression down to 420p (indistinguishable from 720p on screens this size, and indeed on most screens below 32 inches, and the best that's worth aiming for if your source material is a DVD rather than a Bluray) that goes down to around 800Mb for an hour's viewing, so you've room for well over 20 HD movies at that compression on a 32Gb card. The real space gobblers are videos recorded as raw streams from HD receivers or players. These 1080i captures come in at around 8Gb for an hour of viewing, so you would indeed be limited to a couple of them on a 32Gb card, but they're pretty rare beasts anyway.
BTW for just viewing HD videos, the card doesn't really have to be a class 10. It's different if you're actually *creating* HD videos on an HD capable camera or phone. Then you do need the fastest speed card you can get, otherwise it won't keep up with the data flow when it's writing.
I have several hundred video files, most of them East Asian HDTV material, using all sorts of compression and containers, and I've yet to find a single one that this tab (I have the 8Gb version myself) won't play flawlessly. Better indeed than my much more expensive Samsung kit or my dedicated Western Digital HD Net TV player can manage. And when I'm at home, I don't even have to copy them to the SD card. The tab will play them smoothly over WiFi straight off my LAN server (and that includes 1080i material, too). Admittedly, for that to work with 420p or better material, the tab has to be in line-of-sight from the wireless router, although less demanding video material will pass smoothly over WiFi through a few walls or a floor as well.
Coming back to the original question: if you get video freezes on these particular devices when playing from your SD card or internal memory, and you're sure that the video file itself isn't defective, then you either have rogue software running in the background and stealing CPU power (alas that's an all-too-common situation) or you have some sort of hardware fault. The internal video processor is extremely powerful and efficient, and you are very unlikely to have video files that overtax it unless it is a faulty specimen.
Playing video over WiFi or 3G is a different matter. There, freezing and stuttering is nearly always caused by the video processor not being supplied with the data fast or steadily enough. This may be down to wider internet connection problems, or (more likely) to interference with your WiFi signal from the WiFi installations of your neighbours. These tabs also "like" some models of wireless router much more than others. I have never managed to get a sufficiently stable and fast connection for video playing between the tab and our Netgear-manufactured Virgin "Superhub", but that doesn't matter to me because we have a second wireless hotspot on our home LAN that the tab is perfectly happy with.