A USB DAC which supports bitstreaming multichannel DSD is not going to be cheap - the only one I can think of which supports this is the
exaSound e28, and that's $3300+ depending on the options you select.
If you don't mind converting to LPCM, you can use an HDMI cable from your PC. Media Center decodes DSD to 352.8kHz PCM, which it can then resample to any other rate.
If using a cable is an option for you (i.e. the PC is near the AVR) the easiest solution is to simply connect an HDMI cable and have Media Center convert to 24/192.
As for NVIDIA not supporting 88.2/176.4kHz outputs, I would suggest either replacing the video card with an AMD one, or simply upsampling those rates to 192kHz. Anything else can be played at its native rate, and in my experience 88.2/176.4kHz content is rare.
And if an HDMI cable from your main PC is not an option, using something like an Intel NUC connected to the AVR would be able to receive and play 5.1 from your server PC.