Pass Audio from ATTUverse STB to sound card optical to analog speakers

Soon i am going to be getting a Samsung T260HD and am going to be hooking it up to my computer for an upgrade to my current monitor and hooking it up to my AT&T U-verse Set-Top-Box (STB) to replace my old tube TV. I have a reasonably new set of 5.1 analog surround Altec Lansing computer speakers and i would like to pass the audio from the STB to my computers Sound Card via Optical Cable (TOSLINK) then to the analog speakers via the sound cards 3.5mm jacks (1/8" ). FIRST...CAN I EVEN DO THIS WITH MY SET-UP??????????????????????????????????? I know that it wont always be 5.1 but if i am watching an hd channel i want to be able to hear it in 5.1 instead of the crappy built in speakers that the TV has. I need to find a good sound card with atleast Optical Input and 5.1 Analog Output and be PCI or PCI-E. It would be great if it didn't cost more than $140 but a little over is ok. Also if it could be from a well known company like asus, creative, ect.

THANKS

Here is the order again: AT&T Uverse STB----via TOSLINK---->Sound Card----via Analog---->5.1 Analog Speakers

The Creative X-Fi Titanium Fatal1ty Pro fits my needs but does anyone have a better card (similar price) or one they liked better...PLEASE TELL