There are two techniques to project sound from a PA system within a hall, either 'distributed' or 'focal point'.
Distributed sound is simply the strategic placing of many speakers throughout the auditorium concerned such that any point is no more than a few metres from any one speaker. The only criteria is that any one point must also not be able to 'see' the face of more than one speaker unless there is a vast difference in distance between the two or more faces seen so that the closest speaker is the only one heard by the listener.
As can be imagined, this can be quite a feat in a rectangular hall. The other problem is the direction of the sound is not necessarily the same as the visual i.e. the podium or stage is to the front of the listener with the sound emanating from the side. This leads to disorientation and mental strain which in turn leads to lack of concentration.
Focal point projection eliminates the above issues by having the visual and aural sources emanate from the same direction, this being a more natural scenario. There is one drawback with this technique being sound propagates at a far slower rate than light but the effect is not that noticeable as by the time the delay is marked the visual source is usually too small to notice any synchronisation problem e.g. the lips are too small for the movements to be seen before the words heard. The fact the sound is coming from the same direction as what is being looked at is far more restful to the listener than distributed sound.
There is one disadvantage to focused sound being the sound level nearer the source is louder than at the furthest point. This can be counteracted to a large extent by mounting the speakers as high as possible and also facing them towards the most furthest point. A more even distribution is created as those nearer the speakers are only subjected to the "side blast" with those furthest away benefiting from both the speakers' "angle of max radiation" and the natural attenuation of the air (i.e. not deafened!).
The example shown above was a very real situation where the speakers were originally distributed along the front wall. The audience was known to lose concentration on a regular basis from mental strain owing to the sound approaching from the wrong angle. The new configuration proved exceptionally successful.
There are a few points to be aware of. The alignment of the speakers must form the arc of a circle such that if a perpendicular line was drawn through each speaker they would meet at a central point (i.e. a focal point). It's amazing how many so-called 'experts' get this one wrong. You want proof? Have a look at the speaker set-up the next time you see/visit a rock concert.
All speakers should be at the same height (as can be seen in the example with the centre speakers mounted on extension rods). Different heights (as does bad alignment) destroys the focal point possibly causing 'blurred' sound. The more accurate the focal point the better the sound quality.
So How Much Power
When the hall is empty then you may well need no more than 10W to hear everything clearly. However, in the very same hall pictured above, as soon as the hall fills with people wearing all that sound absorbing material (especially in winter) then the bank of 100W amps were working so hard I could have fried eggs on the back!
It clearly depends on what the PA is going to be used for. If for speech, then relatively little power will be needed (about 10W per speaker is a fair guestimate), however, if you are wishing to do stage productions then I would not settle for anything less than about 300-400W total power.
What I genuinely can suggest is that you hire a decent piece of kit and try it for what you reckon the system will be needed for. By the way, don't forget a decent equaliser! And don't let anyone tell you that it should never be changed. The settings will depend again on what you are pumping out through the speaker cones (and what's absorbing it!).