If you use a DAC, it will only be good for 2 channel audio signals. If you have a movie file that has a DTS or any surround sound audio track, you will hear ungodly static from your DAC because it cannot process the signal.
Essentially, don't go with a DAC unless you only want to listen to 2 channel music. I assume your HTPC will be for movies, so you must use a receiver that will be able to decode the DTS, Neo6, Dolby Digital, or any of the other surround sound audio encoding.
Some receivers will have an Analog In where you can use the DAC for its intended function of processing 2 channel audio. But, you would want to set this up as a secondary audio option for when you are listening to 2 channel music. In my opinion, the nuForce uDAC 2 is a good DAC, but it's not worth the trouble to hook it all up and buy extra interconnect cables to bypass the receiver's DAC. I would keep the uDAC as a headphone amp and DAC at your PC and go strictly with the receiver.