Not sure if it is possible with DropBox. I have dropbox on my EVO and in order for a file to show up on my home computer, I have to open the app, then upload the file to the cloud. Then it syncs with my computer. As far as I know, there is no way to monitor a folder and auto update to the cloud.
It does however work the other way, if you add a file to the Dropbox folder on your computer; it auto-syncs to the cloud and will show up in the Dropbox app.