You haven't given much background info, but generally, if you want good quality multi-channel audio from a PC your best bet is to use a professional PC audio interface/DA converter. M-Audio makes a few such products:
4 in/out USB interface -
[Link: m-audio.net]4 in/out multi-channel sound card -
[Link: m-audio.net]They have other products on their web site that you may find suitable. Are they affordable? That's for you to decide, I suppose.
I've never done this, myself, but it may also be possible to install a second stereo sound card (would be the cheap solution). Good luck ironing out any I/O and IRQ conflicts!