The world of home cinema can be more involved than putting a man on mars. First off check out your room. If its not the right shape, acoustically perfect, your search for perfection is a waste of time and that is the position most of us are in. So choose the size/picture quality you're after and buy that gear and then find the size and quality sound system you'd like and can afford. Try to keep as much of the sound from the same manufacturer and proportional in size/output. You should be ok.

Getting into the science of home cinema will do your head in and your wallet and you'll end up chasing ghosts. I have 11.2 capability and there aint nothing to utilize it and by the time it does my system will be obsolete. I reckon my extra speakers do make a difference like I might hear a bird tweet through my front highs and thats it for the movie but can't be sure. So its probably wank factor. 7.1 is all you need at most in my opinion unless you like tinkering like me.   

Does having two subwoffers make a difference?

I will prorbably go for a 9.1 system
later in the year get new main speakers and use my current main ones for the rear


Thanks