tapestries and posters help with sound absorption?

Experimentation is always the rule. Unless the room is very live acoustically you can easily overdo it, so proceed judiciously. Begin with a large thick floor rug centered around the listening area; may not want to cover the entire floor. Wood floors are beautiful & too much carpeting may make the room too dead anyway. Hook rugs / thick tapestry / deco blankets / your foam posters hung at strategic locations (walls directly behind speakers, first relections points etc.) work well. Diffusers or absorbtion on ceiling at reflection points are helpful (I sprayed my entire ceiling with 1" thick cellulose insulation fiber - the bleached white variety - works fantastic acoustically). Slap echo from the wall behind the listening position should be approached a bit more carefully, because too much there (as in a thick blanket) may absorb excessively. Experiment with thinner & smaller materials if too much absorbtion is noted. That compressed foam or tapestry might be ideal. More sophisticated devices include Argent Room Lenz, ASC Tube Traps, RPG diffusers, corner & triangular pillows, Room Tunes.
If you really want to do this right, consult with member Rives regarding professional approaches.

The material Lugnut has stockpiled is called rigid insulation. It is fibreglass and is used for thermal and acoustic purposes. It is good stuff, comes in different thicknesses and densities. Another product I use is called duct liner. It is rigid fiberglass insulation with an acoustically transparent black surface that keeps the fiberglass fibers contained. it is used in air conditioning ducts for sound dampening. it is anti-microbal as well, as if that is important. You should be able to find any of this stuff in the yellow pages under insulation.

GMC/Lugnut Thanks for the info. I am concerned too about airborne fiberglass fibers in the air and that's important in considering the material. I had built some room treatments in my last house with 2/4's and fiberglass insulation batts and noticed a lot of dust in the air.

For anyone interested I just ordered and have received plans for homegrown acoustic treatments from decware. Blueprints and have bought a LOT of MDF both 1/2 inch and 3/4 inch. I'll let you know how it turns out. For the curious:

http://www.decware.com/room.htm

I have to say after reading the white papers I understand these issues more than I have in 20 years of "hobbying". It's time to get serious about acoustics-the level of equipment we have is not to be fully realized until you address your room and resonances in your equipment.