i mentioned this in the other mp3 player thread, but if storage is your main concern i'd go for the rio 500. it has 64 megs internal plus an expansion slot for smartmedia cards (up to 64 meg smartmedia). sure the rio 600 will be able to hold 372 megs with the backpack expansion, but that won't be available for awhile.
128 megs for a portable mp3 player the size of a pager is enough for me.