Your paralell walls will cause certain frequencies to compound themselves causing "standing waves." If nothing is done to treat them, it will sound either boomy or glaring or both in your room to the excess of these frequencies. The best way to treat this would be to put some type of material on your walls to either absorb or diffuse these "waves." Two pro companies that sell this type of product and also can give you much more information about acoustics are:

www.auralex.com
www.realtraps.com

Both these websites offer very useful information about setting up good listening rooms.

One other question on a semi-related topic. Have you thought about a screen for your projector? Good home theater screens can be really expensive. You can try the DIY route to save some money. Do a search on this website for more info and projects:

www.avsforum.com

Good luck!