Moving 'Sites' folder to Dropbox
You could just move the Sites folder into the Dropbox folder - and then make a link/alias from the new Dropbox location back to ~/Sites.

After you've moved the Sites folder into Dropbox you right click on the Sites folder in Dropbox, and then choose "Make Alias" and move this alias into ~/ (remember to rename it to Sites) - alternativ create the alias this way: Can I create Windows-style alias in Finder?