I guess it comes down to how much money you want to spend. As far as 5.1 setups the only 2 I would consider getting would be
Videologic DigiTheatre DTS ($579.00) Plenty of reviews
here
or the Midiland S4 8200 ($300.00) Plenty of reviews
here
One thing about the S4 8200's is that if you play any games they will only play sound in the front sats and sub which sucks even if you only play games once in awhile.
Personally if it was me I would get the Philips Acoustic Edge sound card with the S4 8200's because its cheaper and with the Edge it plays all games, music and movies in full-accelerated true 5.1 surround sound. Which is pretty kewl. Plus you can pop in some DVD's and get some true DD also.
Myself since I don't care about 5.1 cause I watch all my DVD's on my Home Theatre I'm going with the Edge and Videologic Sirocco Crossfire's. Now that will be sweet.