I'm thinking about installing Ubuntu Desktop, but I don't know what flavor is the better for my system. What are the minimum and recommended hardware requirements? What kind of CPU? How much memory? Should I have Hardware Acceleration? What flavor should I use?

This is an attempt of a canonical answer. My answers have the "official minimal requirements", the recommended are a mix of official sources and opinion based (along with the answer it's told the source). You can comment or edit if you feel that the information is obsolete or incomplete.

Is a good rule of thumb that any system capable to run Windows Vista, 7, 8 x86 OS X will almost always be a lot faster with any Ubuntu flavor even if they are lower-spec than described below.