Yea but I my macbook air only has 64gbs. I think an external hdd would be best right? Or would iTunes match be best?
Oh, OK -- so you can't bring your
entire iTunes library over because it won't fit on your Air.
If I were in that situation, I'd consider either using iTunes Match, or the Home Sharing.
Home Sharing is a free feature built into iTunes. As long as your Dell laptop and MacBook Air are on the same network (and you've enabled the Home Sharing feature on both), you can select songs/albums within iTunes on your Air, and they'll copy over from the Dell. To me, the advantages to Home Sharing are that it's free, and since you're copying from one laptop to the other, it'll probably copy the music more quickly than with iTunes Match.
iTunes Match's advantages (to me) are that you won't need your Dell laptop to be around in order to copy music. When you sign up for iTunes Match, it'll upload a list of all of the songs/albums from your Dell computer up to iTunes Match, and you'll see that same list on your MacBook Air/iPhone/iPad. Any song that doesn't already exist on your Air/iPhone/iPad will have a cloud icon next to it, and if you click the cloud icon, it will download the iTunes store version of the song/album to the device from the Internet (which is why you don't need your Dell laptop present for this to happen). Depending on the quality of the song on your Dell, the fact that the iTunes store version gets downloaded may be a good thing (it may be higher quality), or it may be a worse thing (it may be lower quality, if the version on your Dell was ripped at a very high bitrate). From reading posts on MacRumors over the last year or so, it seems like for most people, it's a good thing.
If you're interested in playing around with Home Sharing before you make the decision to purchase iTunes Match, Apple's got a pretty cool page for it here:
http://www.apple.com/support/homesharing/
I could be wrong but since Wintel HD and OSX HD have different organizations, I tend to believe it doesn't exists, that means after you copy the files over, THEN you will need to again import them, build, your Mac iTunes library from scratch.
As long as all of the media is stored
within the iTunes folder, there isn't a problem copying the iTunes folder from Mac to Windows (or vice-versa).
If all of the media ISN'T stored within the iTunes folder, all of the instructions I've seen have the user consolidate the library (i.e. have iTunes move the media to within its folder) before copying the folder.
If you're curious, Apple has a guide for it (under the "External Storage" area) here:
http://support.apple.com/kb/ht4527