Move iTMS Files

Okay, I'm selling my iMac on Ebay to get a 17" Powerbook. The iMac will be gone by the end of next week, my PowerBook might not arrive until the end of the month. I have an external firewire drive on which i am going to backup my entire HD, but I am not quite sure how to copy the iTMS songs. I know that you can authorize them on a second machine, but I don't know how one would do that, especially with the original computer not around. Additionally, I would like to know if I can de-authorize my Mac before I ship it or if that would void the songs, as there wouldn't be a computer that they are authorized on.
Next big question mark is why I bought the MX900 on Amazon yesterday, when today I can get it for €8 less. I just don't feel like sending it back, though since that would just not be very nice. But I think that's not really part of the discussion here, it just annoys me :-( Well, at least my broken iPod (also on ebay) is at €52 right now. €52 for trash. Thank you very much.