Think of it this way - the normal antenna on your router sends the signal in every direction. This Antenna focuses the bean in one direction and you pick up some gain. Will you be able to get 600ft? I bet you will if you have line of sight and two of these (one of the router and one ont the bridge facing each other) you will get 600 ft. However if is not a direct line of sight with trees/walls in the way hard to say. You could buy one antenna try it and then buy the 2nd if it does not work.
Other options are a yagi antenna. Do a search on a yagi antenna. They give better gain, and if you are handy you can create one yourself. Get two of them and point them at each other and you can go a mile or so. You might want to look at increasing your routers performance. Look at the forums from dd-wrt.com for information on that.
Another option might be the powerline products that run your network over the house electric wires. If the two locations share the same electric feed and wiring then these type products work really well and really easy to set up - just plug them in and go.
http://www.newegg.com/Product/Product.aspx?Item=N82E16833156234