Supplementing the Sound with a Subwoofer
If you go with bookshelf or satellite speakers, you'll definitely want to add a subwoofer to the mix. A subwoofer, such as the Polk Audio PSW303, is a powered speaker that contains a massive woofer. It's designed to handle very-low bass frequencies, and contains its own power amplifier to do just that. (That's why it's called a powered speakerit has its own power.)
These low frequencies are not directional, so you only need a single subwoofer no matter how many audio channels you have in your system. Because of the non-directionality, it doesn't matter where in the room you put the subwoofer; you'll hear the bass no matter where the speaker is.
You can also use a subwoofer with floorstanding speakers, and probably need to do that when you're listening to movie soundtracks. That's because Dolby Digital and DTS soundtracks both incorporate a separate LFE channel, which is fed directly to the subwoofer. (It's the ".1" in 5.1-channel sound.) You can run floorstanding speakers al fresco when you're listening to music, but you'll need that subwoofer when you watch your favorite DVDs.
Alternatively, you can purchase a four-way speaker system with a built-in subwoofer, and split the LFE channel to each speaker's subwoofer. It sounds more complicated than it is, fortunately.