Its going to depend on your budget really. You can go basic or "sky's the limit".
If you're on a budget, go with these.
If you want some booming sound, go with either of these two.
Now, if you want to have awesome sound, and you have room for home theater components then you can do what I do. I run the audio from my sound card to my home theater receiver and do 7.1 through my big speakers (then again, my TV is also my monitor).
I would recommend you stay away from HTIB (Home Theater in a Box) systems as they have very lackluster sound quality. They lack fidelity with highs and lows, and overall just sound tinny or muted. If you have your own place and you want to go for the gusto, get yourself some good tower speakers with a high quality center channel, and then some good bookshelf speakers for your rears. You can very easily go 7.1 as well since most PC games will support whatever audio setup you have is you have a sound card able to process the audio properly.
Here is a quick list of my setup if you are wanting to go the Home Theater route
Denon 2807 Receiver
Two Definitive Technologies BP2006 Towers
One Definitive Technologies CLR2002 Center Chanel
Four Definitive Technologies BP2X Surround Speakers
One Definitive Technologies Supercube II
Panamax m5825 (I think) Power Conditioner
Panasonic tx-p50gt50 TV
"Follow your dreams. You can meet your goals. I'm living proof. Beefcake, BEEFCAAAAAAKE!!!" - Eric Cartman