Smrex,
I think you've got it. I'm not familiar with the DAC inside the Oppo and the inputs to it you have available. Regardless of which of the two you use, you will likely get an improvement with a good (no need to be the best) USB to SPDIF converter.
Neither am I familiar with the LaCie drive mentioned.
But they sound good conceptually.
BTW, the USB cable from computer to DAC or converter matters much more than the firewire to the music storage. A generic USB will work, though, to get you started.
Have fun!