I've been using a Sanyo PLV-Z5 for quite a while in my theater but since it's only 720p, it doesn't look that great when I'm watching Bluray movies, etc. I'm sitting about 15' from a 110" screen so according to this chart 1080p will definitely be noticeable over 720p.
Anyways, what is the "cheapest" ($1k - $2k?) upgrade to 1080p from my current 720p projector?
Is 3D an option for this price-range?
I'd absolutely love to get a 1080p, passive 3D, projector but as far as I know they are $8,000+, right?
Passive 3D projection requires a special screen and either two projectors or a lens attachment so yes will likely be out of your budget.
But there are active 3D projectors in your price range.
Optoma HD33 (DLP), Acer H9500BD (DLP), Epson 3010 (LCD) and sometimes BenQ W7000 (DLP).