Planning Before Starting a Project

Question

How much planning should one do before starting a project? Should they have everything already planned when they begin coding or should they just get a basic idea of what they want and then make things up on the fly?

For instance, I want to create a YouTube client that allows for streaming videos and for downloading multiple videos simultaneously (similar to Minitube). I know what I want the interface to look like when the program is first opened. Is this enough for now? Should I create this and then plan the next step or should I continue planning?

How much planning is enough?