eg. if you target 3 word phrase "buy books online" and your domain is buybooks.com
which url would you use

buybooks.com/online
buybooks.com/online.html
buybooks.com/buy-books-online
buybooks.com/buy-books-online.html
buybooks.com/buy_books_online.html

examples 3,4,5 have 'buy books' keywords twice in the URL. also a directory or html page? I use mod_rewrite, so it doesn't matter whether I add .html to URL or not, contents are pulled from plain text file anyway.