you're looking for SPDIF as jay said

http://en.wikipedia.org/wiki/SPDIF

the only card i know of off hand is this
http://www.rme-audio.com/english/hdsp/hdsp9652.htm

but you're talking big bucks and it only has 3.

why exactly are you looking to use digital connections? the only real gain to be had (outside using it for sync) is noise shielding from mechanics such as cd motors.