additional hard drive, Adding a second hard drive Options

[Maccabee]

I have an old computer that I have been using as a test computer but I have nothing to test, and i recently got a new desktop and I want to take the hard drive out of the old computer and put it into the new one for extra space. I know how to do it basically but I have some questions. First, should I format the hard drive of the old one, cause it currently has ubuntu on it and it has no files I need. second, I have a lot of open spaces for hard drives and there are a lot of power cables but there are no spare ide cables. And if I took some from the other computer where would they plug into. And lastly how would I make the new hard drive im installing the secondary one. And not effect stuff. Im pretty sure its called the slave hard drive actually. Thanks.