Quick Question....if I get a tablet with 512MB and Cortex A8, would I be able to play 720p mkv compressed videos file smoothly? If yes, can 256MB do it too?
Video playback is usually offloaded to a separate processor. If the video processor cannot handle the format, it will not play. It varies depending on the platform, and also depends on firmware support.
Tegra 2 tablets generally can support it.
My answer is specific for the 8 inch Cortex A8 you probably had in mind.
If you ask on a per device basis, this can be answered.