Yup. That sounds correct. I assume that you're going to do a 4:3 screen since the projector element is 4:3 native? Otherwise, isn't most of the material you watch in 16:9? For me, I'm doing a 16:9 and the rarely used 4:3 will have to fit in the height of the 16:9. Of course I'm using a CRT projector and I'm not losing much in the way of resolution in 16:9. For a 4:3 native element (such as yours), 16:9 looses a lot of resolution, and then on top of that, a 4:3 window inside the 16:9 window, which is sitting inside the 4:3 native element is A LOT of resolution loss. The only option I would suggest would be a 16:9 optical lens for your projector. That would allow you to run EVERYTHING at the full 4:3 native resolution, including anamorphic DVD. You would have a lot more resolution of your image, and it would be brighter too. Not that light is an issue with your setup.
So the bottom line... unless you plan on getting a 16:9 anamorphic lens, you should go 4:3.
It's all about your perspective.