The Audigy series from Sound Blaster, has several different cards in different price renges, that do 5.1 and up to 7.1 surround.
I have the Audigy 2 ZS Platinum. It costs about $200 at Best Buy. Probably find it cheaper online. It handles up to 7.1, and sounds great. I think they have a 5.1 card at Wal Mart in the $50-$60 range.
Good luck! Let us know what you end up getting, and how you like it.