I've had the Logitech Z-5500's for probably about 18 months now, and they are still probably the favorite part of my PC. They're not perfect, but you get a lot of sound (and decent quality sound) for not a whole lot of money.
I'm running mine with onboard sound on a pretty high quality motherboard and think the sound great over all. They highs are actually better then I expected from the single driver sats, mids are rich and come through strong, and the bass is very powerful. The bass isn't as tight as it could be in the big ported box, but it's not by any means what I consider muddy either. It never sounds garbled during fast drum/bass beats, but it certainly isn't as distinctive as the sealed 12" Audiomobile MASS subs I have in my car either.
They are rated at 505 watts, but at something like 10% distortion, so it's misleading... but that being said, the Z-5500's get uncomfortably loud well before you reach full volume. Really the only time I use anything over 3/4 volume is when I'm showing them off to someone... there's just no point for me going higher then that as that level is already loud enough to leave my ears ringing.
With bass-y tracks and the sub turned up passed the default level things quite literally shake off shelves and my desk. I've knocked down more then a couple pictures since I've gotten these speakers much to the displeasure of my wife. For a single 10" woofer it holds it's own pretty well.
I really, really like these speakers for gaming. I use the direct outputs on my current motherboard (used optical on my past board... I don't know what the difference is other then optical seemed to be louder at a given volume level) and let the Z-5500 do the decoding. The different channels come through very disticntly. Before these speakers I had an average 2.1 setup, since getting these I don't think I'd want to go back to a 2.1 setup for gaming. The 5.1 really adds a whole new element to gaming for me, the surronds and seperate speaker channels really add to the atmosphere of games.
I know you said you wouldn't use them for movies much, but for whatever it's worth you may want to watch a movie or two once you get these.
Anyway, I have these speakers and really like them. You can certainly do better for more money by getting a receiver and buying good quality speakers all around, but these are hard to beat at the $250 range. They have lots of imputs also. My wife keeps her MP3 player connected to them so she can listen to music when she scrapbooks if I'm not using the PC.
Anyway, good luck with them if you do end up buying, though I'm sure you'll like them.