Best audio option: invest in a modern video card and an AVR which can be interconnected using HDMI for both audio and video. (Displayport to HDMI adapters are readily available, if necessary.) This provides full multichannel lossless audio from high resolution sources.
Second best: Invest in an audio card or USB dongle which provides S/PDIF over coax. This provides compressed 5.1 Dolby Digital multichannel audio. It really doesn't matter if it's a card or USB device. Both provide equivalent sound, unless, perhaps, you have lots of high-throughput devices competing for bandwidth on the USB connection.
Third best: invest in a high-quality audio card or USB dongle with multichannel analog outputs. This introduces additional conversions (DtoA in the computer, AtoD in the AVR), but you shouldn't be able to hear the tiny distortions this might introduce. However, room equalization can't be applied by AVRs to their multichannel analog inputs.
Last choice: stereo analog connections. This limits you to matrixed surround sound. Room equalization can be applied to this, though.
Analog audiophiles might reverse the second and third options.
There's nothing really wrong with the last choice as a stopgap until you upgrade to the first.
Edited to add: the second choice (coax) also can provide 24/96 stereo LPCM, but not multichannel PCM.