If you're downloading music from iTunes or Amazon, they're all 256 kbps (theoretically). Apple uses AAC, while Amazon has various mp3 standards centered around that 256 kbps rate (variable bit rate, constant bit rate, average bit rate). So if you download from them, you cannot (and shouldn't) change the rate, as resampling from a compressed file only degrades the quality (never improves it).
If you have your own CDs (or if you saved in a lossless format like FLAC), you can always rip them again at higher quality rate. But once again, don't try to resample once it has been ripped/compressed to whatever rate, as that only degrades sound quality. If you want better quality, only way to do is re-rip them again at higher quality rates from the original lossless format.
The rate you rip them also depends on your ear, so while some people say they can hear quality difference between 256 and 320, other people say there's no discernible difference above 192 kbps.
I myself rip my CDs based on the standards outlined here:
http://uberstandard.org/index.html
Its a Lame-based standard for ripping mp3s. I recently decided that for now I will stick with mp3s over AAC (iTunes) because not all my players support AAC, and that site outlines some good practice in ripping mp3s.