How powerful of an Air Conditioner should I get for my 15x20x8 room ?

You should get and air conditioner that is around 8000 BTU's. You can go to a href="http://home-air-care.com/acsize.htm" rel="nofollow"http://home-air-care.com/acsize.htm/a and enter your information at the bottom of the page and it will calculate what size you need.