Use soft links. Hard links make it so that Dropbox can't tell when the file is updated. This is because Dropbox doesn't poll the contents of every single file you have, it just looks at modification dates on the files located in your Dropbox.
This is exactly what I use for syncing my dot files with Dropbox:
$ ln -s ~/Dropbox/dotfiles/.vimrc .vimrc
and vim still loads the soft-linked vimrc file.