300$ multimedia speakers are WAY better than 300$ home theatre speakers. One good point Maximum PC brought up awhile ago. 300$ is top of the line for computer speakers while 300$ is still bottom end for home theatre systems. And my family has a nice setup for movies in the living room, but it is inferior to my speakers, and the setup I have is at least 800$. That being said, yes, home theatre speakers are superior to multimedia speakers. But the cost is completely uncomparative. And a good soundcard+Z5500s vs 300$ reciever and a nice 5.1 system by, for example, polk audio, or klipsch, costing at least 1000$ for the speakers alone, will not yield you any true benefits. I would say most people have there computers in a smaller room, then say, a living room, and don't need to fill up a large space.
In simple conclusion, and I think most people can agree with me on this. If you computer is used in a smaller room, and you just watch occasional movies, play games, music. Then computer speakers are probably just what your looking for.
However, if your computer is, for example, a multimedia system in a living room with a 42" plasma display and you have the cash to throw around, go for it and get yourself some 2000$ speakers. But I promise you that The Z5500s will do the same thing. (Z5500s can get to ear damaging levels with almost no audible distortion. The sub is enough to remove items from shelves and off the walls. And for anyone who thinks different, have you personally listened to Z5500s and equivalent in price home theatre speakers?)
Anyways, I'm going to stop rambling. I will go as far to say as I am an audiophile and I am extremely picky with my audio equipment. And I see no need to spend any more money on speakers because the performance gain would be minimal.