It might help us to know the model of sound card you plan to use.
In my experience it is not possible to connect a speaker directly to the sound card, (unless it is a self powered unit designed specifically to be used for a computer) you need an amp to give the speakers power as sound cards only output signals ( very low power electricity) If you are dead set on not using a receiver then you will need at the very least a multi channel amp or multiple single channel amps. So factor that into your budget as well.
Now for speakers if you act right now there is a very good sale going on at polk... (http://www.polkaudio.com/products/home-theater/floorstanding
) you can get 2 TSi300s for the fronts and TSi 100s for the rears. Those 4 should be a tremendously good start. Then just wait for a center to go on sale or find one used to complete the set. This will be way way way way better than any satellite set with a subwoofer.
Ah the subwoofer... thought I forgot did you? No, if you are living with people who will be bothered by bass then skip the subwoofer until you have a place where you can let it go as loud as you want. Also those towers should supply some amount of bass so you wont be left with nothing...
Or if you really were just looking for the best 5.1 computer speakers in that price range, if you can find a Klipsch Promedia 5.1 system (looking through ebay only found 1 full set) that would be an ok solution compared to the polks.