I use an OPPO UDP-203 to play flac files from external usb hard drive. I think I understand that the oppo´s dac does not support sample rates higher than 192 kHz. Then I think that 24 bit/176400 Hz is the best choose possible?That is a broad and difficult question, it really depends on how well any particular DAC handles PCM playback vs. DSD playback, and your system's ability to flesh out those differences. It should be a very small but perhaps not totally inaudible difference.
Yes it is.