Transfering old files to a new XBox 360

I just won a new XBox 360 "Slim" and I was going to give my old one to a family member. However; I have a large amount of game data, downloads, etc. on the internal and external hard drives.

Now I know there is a cable to transfer the files with, I was hoping someone could tell me what B&M store/online retailer has the best price for it. Also, I know the hard drive is proprietary, but is there a way to find it cheaper than retail?

Also, do I just sign-in to the new XBox with my old gamer tag info or do I have to create a new one and delete the old one from the console before I hand it over?

Thanks for the help!