I just purchased the Cowon D2 and am a little disappointed with it. Mainly the 4000 file limit, not being able to create and save playlist and the sorry random play function.
I have a D2, and as I understand it the limit is 3000 files on the internal storage and 3000 on the external.
The 405 is a perfectly satisfactory unit -- I've been tempted to by one since they're really cheap right now
But compared to the D2 I think you'll be disappointed with the battery life and the sound quality.