Most importantly, It depends on which age group you wish to get in your shop.
I could bang on about what music helped sales when I was working at Our Price, but I don't think that would be relevant to your shop.
The only shop I go to where I have noticed that I actually like most of the music they play is TK Maxx.
Edit. Should have read your post properly. Good music is a good thing in shops, bad music is a bad thing. I wouldn't go over the top on system, as the music really shouldn't be played so loud.