well for a $500 budget here are a few options...
z906 = $300
http://www.amazon.com/Logitech-Surround-Sound-Speaker-System/dp/B004M18O60
the nice thing about a set like this is that it is relatively inexpensive for a decent 5.1 set and that it connects up to an optical port. your motherboard includes and optical port. this means that you dont really need to use a soundcard for good sound.
energy take classic 5.1 set + receiver or amp = $300 + receiver (the $200 or a bit over you have left over can get you one just fine)
http://www.amazon.com/Energy-Classic-Theater-System-Black/dp/B001202C44
note: bottom image of a particular receiver is just to show you what one looks like. what matches your needs may be different. the benefit of going with home theater speakers is that typically they are better than pc speakers. as far as between the z906 and these there may not be a huge difference due to the price but they are very highly rated speakers. a receiver is best for multiple sources so you can hook up your console gaming systems and the like if you have any. for a single source (pc only) you could use an amplifier instead to save costs since you dont need the extra features a receiver gives you.
the third option is to go with an amplifier and a 2.0, 3.0 or 2.1 set of high quality speakers. you could then get the other speakers to complete the set in the future if you want. this will get you higher quality and more powerful speakers but you would be limited to stereo only pretty much. for this setup i'd suggest bookshelf or floorstanding speakers. the benefit of such a system is that you get the best sound in your price level but lose out on the rear channels (until you purchase them in the future if you wanted).
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if you wanted an idea about what i personally use...
showing my receiver, front & center speakers, sub on floor, ps3, battery backup & tower case as well as my tv which i use as a monitor.
showing a closeup of my keyboard & mouse but also shows my center speaker.
showing my movie collection however you can probably spot the rear speakers of my 5.1 set if you look.
my set uses satelite speakers which are the smallest of the three sizes but work out great for use on a desk. most of the bass in such systems is provided by the subwoofer. in the larger systems with bookshelf or tower speakers quite a bit of bass is provided by the actual speakers due to the larger drivers.
i'm quite happy with the set i use as its a huge upgrade from pc speakers. the actual speakers i use however are out of your budget but there are speakers within your budget like i mentioned above.