Assuming that your receiver

Assuming that your receiver accepts SPDIF (AKA optical audio, TOSLINK), make sure the motherboard you purchase has this capability. This will allow you to bitstream Dolby Digital and DTS audio sources to your receiver for true 5.1 surround sound.

Another option would be to use multi-channel analog into the receiver if the receiver supports it. If you plan to do this, you need to make sure you purchase a motherboard supporting 5.1 or greater analog outputs. The possible advantage of this over optical audio is that you can fully decode HD audio soundtracks found in Blu-ray and send them to the receiver. The possible downside to this is that the DAC on the motherboard may not be as good as that of your receiver.

A third option would be to use only the stereo output of the motherboard. This is the worst option and will not give you true 5.1 surround sound. Every audio source will only be output as stereo.

If you’re happy with your receiver capabilitites and performance, there’s no reason you need to upgrade to a new one. It will work the same with an HTPC as any other source device.