If you copy them to your USB-stick and insert into your iTunes library in your new computer like you did on your current one, you still have them.
A number of the songs in my Itunes library are marked as "ineligible for ICloud" (cloud with line thru it icon). These are mostly musical odds and ends uploaded from Youtube. My question is: if I get a new computer, are these tracks gone forever? Or will they still be in my library? thanks