I don't believe that there is a minimum. Their FAQ states only that the card you are upgrading to has to be priced higher. I'd say give it a try and let us know what happens.
I'm am in a very similar situation, myself. I bought the $359 N518 version that came with the free mobo. According to posts on their message boards, I can keep the mobo and step up to a 7900GT (CO or CO-Superclock, got to call for the prices when I get home) for almost no money

I also can't wait to get home and register. Hopefully the queue isn't too big by the time I get there.
-red