I did this last year. I have a 2 story house built in the mid-80's and I mounted a projector on the ceiling of my first floor family room, which is just below a bedroom.
There should be joists between the ceiling and floor of the two rooms you describe. You should mount the projector to a joist. I'm not a builder or an architect, but I can't imagine a standard multiple story house not having floor joists for each floor - of course someone here could probably prove me wrong...
I have smooth ceilings so I didn't have to worry about removing any popcorn or texture material. Before you mount your projector to the ceiling, you should smooth out the area where the mount attaches to the ceiling.
Here's how I installed my projector. It was fairly intrusive, but it was also inexpensive which was important to me... I searched the forums here and bought a recommended (and inexpensive) projector mount off ebay. Then I used a stud finder to find the ceiling joists. I mounted the projector to a ceiling joist and as close to a wall as I could get it (which was about a foot away from the wall that is opposite from where my screen was going to be). I ran all the cabling from my equipment cabinet through the walls and out through an electrical box I installed on the wall just behind the projector. I had to cut several holes in the wall sheetrock and I had to drill holes through the studs in order to get the wires to where I wanted them. After carefully patching the holes, you can't tell that they were ever there.
I made my own screen by simply painting a screen on the wall where the projectors image landed. Then I got some 4" wide fluted molding from HD, painted it flat black, and nailed it up around my screen.
When I finished, the whole setup passed the 'wife test' AND the 'mother-in-law test', so I guess it looks okay.
Hope this helps...
XBL GamerTag: Fiveprime