Question - if you are going to use your computer for mostly audio listening, then why not use something better than midrange gaming speakers?
If you look at the newegg reviews on speakers like the logitech z530 and z5500 speakers, you can note that pretty much anybody who has done some critical listening with those will point out how they are very weak when it comes to details - in fact I have read more than one review which has stated that they are weaker than most TV speakers when it comes to this.
This is probably OK for most games, especially multiplayer games where there isn't much dialog to listen to other than maybe your buddies over ventrilo or something. But for anything else - movies in particular - you'll probably want something different.
Personally I am trying to build my setup to be versatile for pretty much anything, I've tried many different speakers so far, some great sounding, some not so great, but I am really after that bang for your buck setup that I just haven't quite found yet.
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As for your soundcard, I would recommend taking a look at the Auzentech Prelude. It supports EAX 5 and dolby digital live and DTS connect, with future plans for an HDMI add on as well, and has much better analog components than any creative brand card.
Don't buy it just yet though as many of those features aren't there yet. Oddly enough they released it before including digital support in the drivers which raises a possible red flag. Just keep an eye on it and watch the review sites. I myself am keeping an eye on this one as it has the potential of becoming the holy grail of sound cards.
Regardless of which card and speakers you get, just remember that computers are all digital, and create a lot of EM interference with all of the devices running in there. For that reason, I recommend against using analog connections - keep it all digital until it reaches the receiver. Even though DDL and DTSc are a tad lossy, it still beats hearing your hard disk heads clicking away through your speakers like you get with the typical setup that does the digital to analog conversion on the soundcard itself.