I use a Sound Blaster Audigy, check it out at www.creative.com. It has built in Dolby Digital decoding (AC3), a firewire port, EAX Advanced HD for future games, it sounds really good. I'm using mine now with Altec Lansing ADA885s, similar to your setup but I'm moving on to Logitech Z560s next week.
You can get a Audigy on www.pricewatch.com for $50, pretty good price.
The Gametheater XP is a good card also and can be found for $87 on pricewatch. It has alot of inputs/outputs and a external RACK (according to how rob spells it) and has the same A3D/EAX modes the Audigy does besides it lacks EAX Advanced HD and doesnt have a firewire port.
I'm glad I have the firewire port incase i ever need it but all my devices are USB 1.1...
Athlon XP 1700+,KT266A,Geforce3, Audigy.. 'nuff said.