As for how many speakers one needs in a home theatre, in the majority of cases, imo, 5.1(2) or 7.1(2) is sufficient. Beyond that, it's the declining return theory...you may indeed hear a slight improvement as you add more speakers($$) but for most of us it's not enough diff to spend the extra on such a marginal improvement.
As Mike alluded to, a lot of people consider HT-in-a-box to be enough of an improvement over a pair of 10w tv speakers which make up the vast majority of HT setups, I would think.
