Audio Systems: Sound system for a dance area, mackie srm 450, wattage ratings

Question
Hi Wayne: I am working in my final touches to open a small teen club but friends are giving me different opinions about the speakers to buy, I think to play the music from either an Ipod, cd player and a laptop next the signal goes to a 4 or 6 channels mixer and then one powered loudspeaker to reproduce the music monophonic. Now the first question: is it better to buy a powered loudspeaker or a passive one with a separate amplifier?.  2) if the dance area is gonna have 500 or 600 sqft(in door) with capacity for maximum of 80 people, How many watts of out-put I need to play the music?. how can I calculate the wattage of the loudspeakers?
I have been looking in Sam Ash and I like either one of this 2 options but can't make a decision and you can help me to decide:
First- one Mackie SRM 450 V2 12" Powered PA Cabinet with the Behringer Xenyx 1202 fx 12 input mixer(with effects).
Second- Yamaha Stagepas 250M portable PA
I want to thank you in advance for helping me how to calculate the wattage of the music and decide which elements to buy for playing the music.

Answer
My personal preference would be to use the Mackie powered speaker, as these can be used with any system / mixer (i.e. - you don't need to worry about impedance mismatches between speaker and amp).

For as small of a venue as you're describing, I'd say the Mackie should be ok, but as with any single cabinet, there will be "hot spots" as far as loudness is concerned. Just take a few minutes before the event to "tune" the cabinet to the room. Proper EQ'ing can do wonders - it can give the "feeling" of loudness without having to crank things up too much. Use subtractive EQ - that is, if you want more low end, instead of cranking the low freq. knob up, try reducing the high / mid freq's and increase the volume slightly.

As far as wattage is concerned, it's always better to have too much than too little. Keep in mind that the wattage ratings are measured at full volume. Many times, you won't need that much, but having the extra "overhead" will help keep the signal clean and free from distorting and/or clipping.