Obviously not everyone's living room will allow for the "Ideal" setup, but this page shows what to aim for while building or re-adjusting your Home Theater.
Even if you can't have every speaker in your Home Theater the same make and model, you should try to at least make your front 3 speakers identical.
Despite what you might have heard, your "surround" speakers sound better when they're pointed towards the listening position (and, of course, identical).
The picture below shows one of the finest movie recording studios in the world using 5 identical speakers mix and record their soundtracks.
At "Skywalker Sound" they're using Bowers and Wilkin's (B&W) "802D Nautilus" speakers all the way around the room. These hand-made speakers are 200 pounds a piece, take 500 watts RMS each and cost $6,000 retail, for ONE.
With movies being recorded and mixed in the studio with 5 (6 or 7) identical speakers, shouldn't we be listening to our movies at home with 5 (6 or 7) identical speakers?