I suggest not bothering with speakers

. Well not quite...
I have decent quality monitors and an amp instead of desktop speakers, but really even those don't get a look in.
When I play the 360, I always use my gaming chair, I have one of these, an X-Rocker gaming chair:
Now the sound quality is probably not the greatest, but the fact that it's sitting on your shoulders feeding your ears makes a lot of difference, it's much more believable, even though it's just 2 speakers and a bass sub. The chair is comfortable (I'm 6ft and my head sits perfectly on that raised pillow bit, I could lean back and have a snooze quite easily), it's loud, and when playing games like 1943 or Fallout3 I just get sucked in - I actually play better when playing in my chair. So out of my hi-fi amp and monitor setup, my noise cancelling headphones, expensibe Bose in-ears, and my TV output - the chair sounds the best for playing games. Which should be the point - your not listening to music or watching a movie - headphones and earphones are better for music anyway, if you want a really great gaming sound experience, get a chair! - they tend to go for around £80 for one with sound.