Since you have a good A/V receiver, I would start with a soundcard like M-Audio's Revolution 7.1 sound card which is really geared toward home theater enthusiasts. It has the following features per M-Audio's Web site (http://www.m-audio.com):
24-bit/192kHz audio playback on all output channels.
Supports all speaker configurations from stereo (or headphones) up to 7.1
Analog line in supports recording up to 24-bit/96kHz
Analog mic in supports recording up to 24-bit/96kHz
Coaxial digital output
S/N Ratio A->D: 100dB
S/N Ratio D->A: 107 dB (eight channel/two channel)
Driver-Level Bass Management
Windows Media 9 Compatibility
Very low CPU utilization
Dolby Digital Decode (Windows)
Supports both CircleSurround II and TruSurround XT
You can install this card into your PC's PCI slot and use your PC as a music playback device. This will provide much higher fidelity than audio than what you can get from audio built into a PC's motherboard. Most home theater enthusiasts really like this card for PC-based home theater set-ups. You also have the option of going digital out on the card to digital in on your receiver. It would probably be worth it to test which device (soundcard or receiver) has better digital to analog (DAC) conversion.
24-bit/192kHz audio playback on all output channels.
Supports all speaker configurations from stereo (or headphones) up to 7.1
Analog line in supports recording up to 24-bit/96kHz
Analog mic in supports recording up to 24-bit/96kHz
Coaxial digital output
S/N Ratio A->D: 100dB
S/N Ratio D->A: 107 dB (eight channel/two channel)
Driver-Level Bass Management
Windows Media 9 Compatibility
Very low CPU utilization
Dolby Digital Decode (Windows)
Supports both CircleSurround II and TruSurround XT
You can install this card into your PC's PCI slot and use your PC as a music playback device. This will provide much higher fidelity than audio than what you can get from audio built into a PC's motherboard. Most home theater enthusiasts really like this card for PC-based home theater set-ups. You also have the option of going digital out on the card to digital in on your receiver. It would probably be worth it to test which device (soundcard or receiver) has better digital to analog (DAC) conversion.