Speaker placement -
Dolby Labs has some recommendations.
Speaker mounts - Depending on the size, wall and ceiling speaker mounts are available at Wal-Mart, Best Buy and Circuit City. If you need larger ones, then online from Parts Express or some specialty shops.
Projector screen - A lot of people will recommend different screens that ranget from $500 to $5,000. I built mine in a weekend for under $70. There are even paint formulas that you roll on for around $20.
The home theater in a box (HTiB) are nice but not the best choice for a dedicated room. What's your budget on sound? What's your budget on this whole project?
-Robert