You got a few options, I think. The first option would be to create a folder action. I've got no idea how to tell you to do that, but it's an option. What it will do is watch that folder for any changes, and then it will perform that action on the folder for you.
The second option is a program called Hazel. You'll set up a rule, have Hazel watch that folder, and when that folder changes, Hazel will perform the action. It's the same as a folder action, basically, but creating it is more like creating a rule in Mail.
Is there a reason, though, you can't just save stuff directly to the Dropbox folder?