Hard disk full - question...

get a 500gb external and clone the entire drive to it. use superduper. the external will be able to boot and you will have enough room on your internal.

 

No it wont be taking up twice the space. The music/videos are either in the iTunes media folder or in a different folder (one you created).
Go into the preferences in iTunes and click the tab saying advanced. There should be a box you can tick or untick where it says: copy files to iTunes media folder adding to library. If its ticked all movies, music, tvshows etc will be in the itunes media folder (location of that is listed above). If not then the music is stored wherever you keep it

Buy an external HDD, USB 2.0 is quick enough. A 500GB is like 40, so no biggie. Use this guide to copy your iTunes media folder to the external. In order to then listen to or view data in iTunes the external with the files needs to be connected and on all the time.
http://support.apple.com/kb/ht1449

 

Possibly. If you selected "copy files to iTunes Media folder when adding to Library" in the Advanced Preferences tab, you may have duplicates all over your machine. For instance, if you have a folder of MP3 and add them to iTunes and have the "copy" option selected, they will both remain where they originally were AND will also get copied into the Library. It depends on how you set it up initially.

 

You will be running into other problems too having a disk that full. You will be getting a lot of fragmentation leading to the disk having to work a lot harder and shortening its life. You can also have other serious problems arise. I recommend that you make a copy as the others have suggested - hopefully you have a backup in case anything goes wrong - and get some space on your hard drive as a matter of urgency.