How much power do you need for realistic HT?

Well,
You can never have too much clean power!!
The more power or current you are using, the better the sound at lower volumes and the more current delivery at peak you will have. Personally, I use a pair of Aragon Palladiums for my front channel...these are class A upto 100 watts.. kind of a b.s. but anyway they will run upto 1 kilowatt at 2 ohms... My center channel is bi-amped with an Aragon 8002, and the rears are run by another 8002 at 6 ohms each for small monitors...
I have had moments more than a few times that I could not believe what was going on in my room... it was almost embarrasing. Ridiculously good, but I even got the impression that I didnt need all that power or volume.
Anyway, go for the gusto!! Dont break the bank!
Happy listening,
Joe
Custom Audio LLC

How much power you need will depend on the speakers being used, the size of the room and your seated listening distance from the speakers. The less efficient and lower impedance the speakers are, the more power that you'll need. The louder you want it to play, the more power you'll need. If you have a large room and sit a good distance from the center and mains, the more power that you'll need. If you add all of this up and have low efficiency speakers that are low impedance and you have a big room that you sit a good distance from the speakers in, i wouldn't begin to think that 200 wpc was enough, let alone 100 - 120. Then again, if you have a reasonable sized room, speakers that are reasonably sensitive ( 90 db's or above ) and of higher impedance ( 6 - 8 ohms ) and don't expect to shake the house on a regular basis, the amount of power that you mention should be okay. Then again, there are differences in amplifiers even though they may be rated for the same amount of power.

With all of the above in mind, i would HIGHLY suggest investing in a separate sub & amp or a combo powered subwoofer(s). This will take some of the strain off of the other speakers if trying to achieve "thunder" in your living room. Sean
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I think that the 100-120 would be fine for a theater with smallish speakers. Any decent theater uses a powered subwoofer which handles all the serious power sapping explosions. The center (usually small sized) primarily handles dialogue and the rears (usually bookshelf sized) give basic surround effects. The mains typically handle most other sounds act as front surrounds.