Well, 44.1 kHz and 48 kHz are sampling rates-- they describe how many times in one second a digital audio stream is sampled. Quality-wise, 48 kHz is better. 44.1 kHz is the standard for audio CDs, though, and to most people's ears, there will be no noticeable difference between the two. 256 and 320 kbps are constant bitrates for digital audio, describing the amount of data per second contained in the audio stream. The difference in quality between 256 and 320 kbps will most likely be greater than between 44.1 and 48 kHz; I recommend going with the 44.1 kHz 320 kbps render option.