Okay well i have the SBS series, as well as my new Altec Lansing MX5021's.. they cost more that the z-2300, very audiophile. I love them..
And a friend of mine has the X series...
Basically, the SBS are great for the price, very cheap. But the X series is probably worth the extra money if you want to spend it. Especially if you dont want much music, cause they can sound a bit weird for that.. games and DVD would probably be thier stronger point.
I'd say just for the bass power in games and movies get the z-2300's. Also depends what kind of sound you want, if you do listen to good (not rap/hip hop) msic on your PC you could also check out my new Altecs.
The z-2300's are very powerfull, so if you need that it would be a plus, otherwise you are gonna pay alot of money just for power, and if your never gonna crank them dont bother. The chewaper ones sound good on lowish volumes.