i was planning on getting one for the same reason. theres basically no way you're going to find a 60gig model for much less than $600 unless it "fell off a truck." and i know theres even less of a chance of finding a 20gig model at all other than ebay, considering every store i've checked with doesn't even carry them anymore.
i've just settled with VGA cables and the 360 to upconvert dvds for anything that doesn't come out on hd-dvd. granted, its not really high definition, but for having to spend a minimum of $600 to watch movies in HD, it'll do for now.