You know what they say about horsepower and wattage; some is good, more is better, and too much is just enough! Seriously, it depends upon your lifestyle. If you just want background music distributed throughout the house, then you don't need too much power. If you plan on "cranking it up" once in a while, or listen to music with alot of dynamic range, then you'll obviously need more. Use good quality speakers, and good amps/receivers. You'll find that using a good quality amp/receiver that uses discrete transistors in the power amplifier section will have more power than one that uses amp-on-a-chip designs. Even though two amps may be rated for the same wattage. An example would be a souped up Honda engine vs. a Chevy V8. Both may be rated at 200 HP, but the Chevy has more torque.
As far as how much wattage I would recommend, it's tough to say without knowing how you plan on using the system. You'll find that it's pretty hard to find a receiver with more than ~100 watts per channel. Realistically, I don't think that you'll need more than that if you keep the number of speakers at a reasonable number (6 pair or less). Just be sure to use either impedence matching volume controls or a speaker switcher with built in protection. You can also use a pair of large (100w) 2-4 ohm resistors in line with your amp. Good luck!
