again, what is your maximum budget. you list $300+ several hundred. an exact budget would be ideal. also i would suggest buying everything at once since pc speakers come in a whole set and individual home theater speakers may be hard to find later if you dont pick them all up now as they can get outdated.
i would not purchase a $190 soundcard if you plan on exporting sound via spdif as this is a waste of money. if you plan on getting a high end pc speaker set such as the logitech z906 or z5500 this will work however such sets feature spdif inputs which means that you do not need a soundcard at all if your system already supports spdif output.
i would not buy a soundcard at all if you plan on using a receiver. i would use hdmi or dvi to output your sound on the same cable from your video card to the receiver. the only way i would suggest optical out to a receiver is if you dont want to run your video through the receiver for some reason.
here are a few options for you to consider:
optical output on your motherboard to logitech z5500 or z906
-you can get such a set for $400 and they will sound better than low end computer speakers however not quite on par with good home theater speakers. an optical connection would make hookup simple and easy. i've read that some people have issues with uncompressed audio over spdif.
3.5mm output on a soundcard to logitech z550 or z906
-an audiophile level card may be better than the soundcard included inside the speaker set and may offer different/better options. this may or may not improve performance of the set slightly. this would handle uncompressed audio better.
dvi or hdmi output from your soundcard to a receiver and speaker system with video passthrough to your monitor
-your video card still renders and outputs the video while the receiver passes through the video to your monitor and only seperates out the video so that it can process it and send it out to your speakers. hdmi supports uncompressed audio and doesnt have any limitations like spdif does. home theater speakers can be much more powerfull and sound much better than computer speakers.
optical oputput from your motherboard to a receiver and speaker system
-i've heard of some issues with uncompressed audio but this can work.
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now, while i pour myself some more moscato (who knew answering threads was such thirsty work!) i took the liberty of giving you a few images to look at....
this is a z906 set from logitech. $400. not a bad set for the average computer user. far better than $100 and $200 computer sets at least.
this is a z5500 from logitech. $500+ (they are not made anymore and are hard to find). from what i've heard this is the best speaker set logitech ever made and is lightly better than their new z906.
klipsch quintet iv 5.0 speaker set $300 (this is what i personally use i paid $500). this is a good example of a matched speaker set. ideally you want to buy speakers that are meant to be paired with eachother. this set has all of them in one box although some sets you buy individual pieces but they are all from the same series of speakers.
klipsch 450w subwoofer $450, 350w subwoofer $350 (i use a 450 and paid about $400). i personally have my 450w turned way down and it is capable of quite a bit of power in a small room. the only reason i have one is because i got a deal where i paid the same as for a 350w.
pioneer vsx-30 receiver $???? (i use this and paid $500 not sure if you can still get it). this is what i would call a middle level receiver. it has quite a number of nice functions not included on basic receivers however a basic receiver for under $250 is really all you need. typically i would suggest between 1/5 to 1/3 of your total budget should be for the receiver.
klipsch kb-15 bookshelf speaker $250/pair, can get onsale for $100. this is an example of a bookshelf speaker. again, there are many brands to choose from. the second picture is of a kc-25 center channel speaker $250 which you will need if you buy all your pieces seperately.
sony ssf-5000 floorstanding speakers $200/pair. this is a good example of floorstanding speakers. we bought these speakers (one pair) as a christmas present for my dad and they use it on their old tube tv (no receiver) and for the low price (floorstanding speakers can be expensive) they sound rather good and are definitely more than loud enough. they have no dedicated subwoofer (technically the speakers themselves have larger drivers which function similar) but they can shake the walls.
here are a few photos showing different system setups with computers
z906
bookshelf speakers
tower speakers
bookshelf speakers
you've already seen a photo of mine from the front. here is it from the back showing my other two speakers.
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as you can see there are quite a few different choices available if you went with a home theater. dont be scared by some of the prices i listed. the parts i personally used totaled about $1400 however you can create a very nice 5.1 set on a much smaller budget. if you buy used equipment you can easily put together a similar system for 1/3 the cost. for brand new parts i would suggest at minimum around $500 (which is what you would spend on a z5500 anyways) but a larger budget of $600-800 would be easier to work with.
a receiver is nice because you can input from multiple sources easily. for instance i use it for my ps3, ps2 and pc. i also have the option of using my laptop, camera, ipod or other device as an input with a simple swap of the knob. i made the switch to a receiver and audio system about 3-4 years ago and i will never go back. i was without my receiver for about a week recently and was forced to use the tv speakers and it was torture. not to mention i had to unplug cables or swap inputs on the tv which was not nearly as easy as reach-turnknob-relax.
if you do not want to go with a home theater setup there are good computer speakers available for purchase as well. even $200 logitech speakers do not sound all that bad for the money but higher end speakers like the z906 will sound better.
everything depends on just how serious a sound system you want and if computer speakers are good enough or if you want more.
for a room your size satelite or bookshelf sized speakers are fine.
......hope this gives you some more insight to possible options out there.