The Kindle that is rumored is a lighted 6" one or a larger Kindle Fire (LCD screen). Obviously, rumors may not be true, but I haven't heard any rumors of a new Kindle DX. However, if Amazon is going to release a new product in July, it'll probably be announced in June, so you won't have to wait long to know the facts.
To be honest, considering Amazon's neglect of the Kindle DX, I doubt that they are going to release a new Kindle DX this year. Also consider that many people don't like the way the Kindle DX handles PDFs: is writing on the PDF, taking notes, flipping through it quickly, etc, important to you? I think it should be fine for manga though.