How much free disk space do I need to install Lion, and how much will I lose by upgrading?

Question

My hard disk is starting to get a bit cramped, with only 30GB available. I'm about download Lion and I'd like to know if I'll need to free up some space before I install it.

Also, I've read that Lion creates a hidden partition with a bare-bones copy of OS X for emergency system recovery. How large is it? How much space will I lose after installing Lion?