If you are thinking of buying a speaker system based on a 12" woofer, floor standing speaker, be careful. Most people seem to zoom in on the woofer size when buying, but you should look at the quality instead of size. Not knowing your price range, I'll take a shot. You might find a tower system, with 12" woofers for $200, that in the long run will probably sound like crap. You might be better off getting a smaller set with a 8" woofer for that same price that would sound better. In this case the 8" would probably have much tighter bass, although maybe not as much output. Not knowing how much you want to spend, you might be better off looking at small satellites and a subwoofer.
Edit: As Poertner said, if you have an unlimited budget, go for a 12" or larger. MIne are 18" subs, but I paid dearly.