Maybe I'm not the typical buyer but the bump up from 720p to 1080p is very visible to my eyes. The way ebay sellers try to market their 854 x 560 projector as "1080p" gets on my nerves!
cordI have a 1080p projector and I can attest that 1080p vs 720p makes a big difference. When you're dealing with 100 inch screens that pixel difference matters a lot more than your 24 inch computer monitor or even 30 inch TV. (Although hopefully your 30 inch also has 1080).
If you're going to get a home theater projector go 1080p. You definitely won't regret it.
That being said make sure the projector has 1:1 pixel mapping if you plan to display a computer screen through it. Otherwise text is nigh unreadable.
The tiers will give you things like better color accuracy and better blacks. Different projectors in the same "tier" can look very different though so just do some research and get what fits your needs best. DLP is also probably better for movies/games.
(This is a reply to above, Gregory, as well.)
tentantless_voidHow do you find the difference when you are sitting in a projection setting rather than a TV or computer setting (when you are sitting closer).