I like your system! I've found that if the system accepts characters such as spaces and punctuation, it's really easy to come up with a memorable, strong password. There's still the need to substitute a letter or word for a numerical character, but that's not too hard. Chances are excellent that anyone on the internet these days can type a sentence with relative ease, and a favorite quote from a book, or a line from a nursery rhyme, or a memorable song lyric all work really well. Plus the possibilities are nearly infinite... ;-)