I use Chief Architect X4 for the actual floor plans. I use Sketchup to do 3d models for anything I cannot import from the Sketchup warehouse into Chief and for some of the simpler, one-off things like designing the TV stand in an alcove in my living room.
You can use Sketchup for all of it...while it takes significantly longer to draw an actual house plan, a single room is pretty easy to do and the price is definitely right. I only own Chief because architecture is a hobby and I do a lot of house plans for fun or for my friends..since you usually do not get a plan if you are not having a custom home built. Not really needed for a theater and probably not worth the cost, unless you want to do other things with it.
I'd say give Sketchup a shot. Lots of good videos on line and with an hour or two of playing, you'll find you can do all of the basic room design. The rest will be learning the tricks to make it easier to do things like moldings, etc. That and you will be surprised how many brands of TVs, speakers and furniture people have already created for you and uploaded to the Sketchup warehouse.