Although Rob can't condone it because it would void your warranty, remember that it's incredibly easy to fit a second hard drive into the unit yourself (instructions at www.dmoore.com). And you can do it at any time without damaging your current MP3 collection or needing to upload the collection a second time.
My feeling is that you should get the 18gb model now, then wait until you have run out of space on it, then add a second 18gb drive later. If you're still under warranty at the time, you could send it into Empeg and have them fit the second drive and still keep it under warranty. Or you could do it yourself. It's your call.
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Tony Fabris