I haven't found the answer to the sample rate question, though I believe OS X resamples to whatever is selected in MIDI setup if you don't have it set to the exact sample rate of the file.
HOWEVER, from what I can glean from the various internets, the bit depth can be left at whatever setting is the highest bit depth file you have and still play bit perfect at lower bit depths. So, for example, if you have bit depth set to 24 on the Mac and you play a 16 bit file, you are not losing any bits and are still technically bit perfect because the last 8 bits are static 0s. In other words, it doesn't matter. Conversely, of course, playing a 24 bit file while the bit depth is set to 16 will result in data being truncated.
So, I feel relatively comfortable leaving the bit-depth on my 'puter at 24. Sorry if all of this is obvious, but I thought it shouldn't be done.