couple brands
for 5.1 computer speakers, you can't go wrong with logitech, creative, and klipsch.
the logitech 5500's are the most feature filled. they did a review here, so read that to get an idea. 10inch sub, 3inch sats. cheapest of the bunch, bang for the buck.
the sound card you have and do the 5.1 for you, so you don't need a decoder, so the creative gigaworks (5.1 not 7.1) will work, and are know for being the best music computer speakers. pricey, but i would pay the premium, as my logitech 2200's have peaky frequencys.
Klipsch have the promedia ultras. have dual 8inch subwoofer and 5 sats. no decoder but you can buy one extra for $150us. these are the most expensive, but have been know for having the best subwoofer.
hopefully this gives you an idea. these are what i would choose from.
PS: you can buy a digital decoder for the creative as well, but more $$.
sheep