I just got my new Paperwhite a couple of days ago, and after still having 195 unread book on my old Kindle 3 (which represented only a part of all the unread books I have), I was determined to keep the book count on the Paperwhite low.
A few hours later, I realised I'd already put a good fifty books on it...
Sigh. Good intentions, and all that. I expect it'll end up just as cluttered as my K3 was, simply because I can never decide what to read next and I have a lot of books that sound about equal (i.e. "things that sound sort of interesting", not "omg must read now"), and I constantly change my mind about what I want to read next, so I do prefer to have a reasonably big selection of different things at hand.
I keep all my books in Calibre, so at least I've got good about deleting/removing the books I've read from the actual Kindle immediately after finishing them, with the exception of a handful of books I always want to keep at hand.