Will a high quality preamp really make a difference in sound?? Also speaker cable

Will a quality preamp make my system sound better?  And speaker cable...seems like everything I read people are split on this subject. My local hifi store brought up a good point; they said that the only thing between your amp and your speakers are the cable. Very good point and I'm inclined to believe it. Don't want to spend a fortune but don't want cheap cable either. Thoughts/ recommendations? Thank you in advance.