When I used to build disco equipment, a good rule of thumb was "two watts of speaker for each watt of amp". That's a wise rule if you habitually turn your amp up to the limit and then some! For home use you can relax it a bit but don't use speakers smaller than your amp's power rating.
Sorry mate; but if that's how you used to design PAs then you must have made some pretty terrible noises..
That rule of thumb flies in the face of every PA I've ever designed, worked on or that you will hear in a pro environment.. these systems operate on pretty much the exact opposite; you want 1.5x the amp power ( at least) to cope with the transients.
The cabinet's rating is pretty useless actually.
3) Volume depends not only on power but also on efficiency. Paradoxically, the higher the sound quality of the speakers, the less efficient they tend to be. Efficiency also drops with cone size. For what it's worth, my 15W 8" speakers (in 1.5 cu ft bass reflex boxes) can put out disco level sound in a living room.
Sorry to knock you again, but No, they don't.... (both the sound quality bit and the disco level bit..)
Back to the OP question; what you need to check is:
How much money do you have?
How many speakers do you need?
You can then work out the total wattage of the system; (add all the speakers up); and how you can wire it up to keep the impedance within the amps range. (tell me what impedance they are and I'll work it out coz i'm nice like that).