I guess the answer will depend on how serious your media room will be. Magazines like Audio and Audiophile now regularly carry articles on sound room design, length to width ratios, sound reflection and absorption, etc, etc. Also, audio.rec.com carries a wide array of research articles, bulletin board and expert discussion on media room design and equipment.
As a general rule, you don't want two hard and parrallel surfaces facing each other, such as two hard walls, a hard floor and ceiling, etc....as this will often lead to standing low frequency waves..very annoying. Also, you will want to finish walls and ceiling so as to minimize sonic transfer to adjoining living spaces. There are special sound absorbing fiber-boards to go under drywall and z-brackets attached to studs that you then attach your drywall to that effectively dampen the walls ibration. A staggared stud double wall with separate bottom plates is a very effective way of isolating sound. Many people seem to think that fiberglas insulation is a sound deadener....but it really isn't....it just doesn't have enough mass and will only mute the highest frequencies.