Projector Central is a good resource. In addition to the projector size/distance calculator, they have others for riser height/distance, perceived resolution vs distance, and I believe, audio first reflection points.
Projectors are still in a state of heavy change. Prices are coming down and there are some major improvements in anamorphic and 3D in inexpensive projectors. I am planning on staying with my 2 year old Epson 8100 for another couple of years while things settle down a bit.