I'm sure that are DE's that work well with less than 512MB of RAM, but there aren't any modern web browsers that do. They're going to put 512MB as a requirement no matter what the DE itself requires.
My parents old Athlon laptop with 512MB runs Linux Mint Mate and is fine for their needs. When they started to complain about the browser slowing down, I downgraded them to Firefox 13, and they were happy again. It's not ideal, but modern enough for reading the news, emailing, and watching videos.
