They have a 20GB model coming out.. but they have a 6GB model now for only $299. Some people have swapped in larger HDs also. Can be used as a portable HD too. [ www.archos.com ]
I don't have one.. but if I was in the market for something like that, I'd probably pick one up. I like the fact that you just hook it right up the USB port and access it like a HD. No stupid software or drivers to load.
6GB is plenty for me.. my MP3 collection is only about 3GB or so.