The best way is to get an external hard drive as mentioned (ie a 120 gig or so). Copy your music folder from your current c drive to the external hard drive. Then just plug the external drive into your new computer. Download I-Tunes and have it draw the songs from your external hard drive. You don't need to copy them again to your new computer (unless you have a laptop and need to take the songs on the computer with you, in which case you will want them on your new c drive). Leaving them on the external drive will save space on your internal drive.
You can use the ipod as a hard drive if you want. You just plug it in to your old computer, let i-tunes start up, go to your i-pod options and make sure "use as a hard drive" is clicked, and then close i-tunes. It will appear as a new drive in "my computer." then you just copy what you want. Do the reverse when you put on new computer and then delete it off your ipod.
Confusing, but not really.