There are many projectors that produce a very high quality image including the BenQ. I personally think BenQ may have some reliability issues that are fairly serious, but that's more my opinion than a fact about the product. In a head-to-head shootout I would believe that the 8720 would pretty handly look better than the Z4 or AE900. But, there are other projectors that are almost brand new designs that should be considered as well including offerrings from InFocus (IN76) and Optoma (HD72).
You really are going to want to consider that screen size seriously. In practical terms, to your primary seating postion, following THX standards, you don't want a screen any wider than .66x your seating distance. So, if your seating is 15', then your screen should be abour 120" wide. This will give the center seats the exact same feel as sitting in the middle of a movie theater. The front row will be like being closer to the front seats, and the back row will be like sitting more towards the back of the theater.
It is important to realize that you will likely see some visible pixel structure if you go any larger than .66x seating distance for your screen size. As long as you know the reason why and understand it, then you can use whatever screen size you want, but the tendency for people to go 'as big as possible' is not good. You want to be comfortable when sitting in your home theater enjoying your movie.
