is a subwoofer for music a good idea??

i have standmount speakers which go down to about 45-50hz.i feel my system lacks low end punch,specially at lower volumes. now i need to know if:
1. a subwoofer for music is a good idea in the first place
2. will buying a budget sub($400) really add sufficient low end
3.is there much info in music below 40 hz
4.will i lose imaging if i add a sub to my 2 channel set up

all you experts out there, plese help.