low disc space!!!!

OK, so I keep getting this message balloon that says my disc space is getting low. Guess I've just got too much music and stuff on the old laptop. Therfore, I have a few questions......can I store all my music in an external harddrive in a way that it would be accessible to itunes? Also, on my comp, I have the Limewire library, itunes library, realplayer library, and also windows media player library. They all have pretty much the same stuff on them. I'm hoping I can make sure everything is in itunes and then delete all the other libraries contents....will this free up more space? Please help! I don't want to stop downloading music...........ever.

Also, it would be great to have all my downloaded files go straight to itunes......can someone show me the easiest way to do so? Thank-You so much!