The ear can't really tell the difference between anything around 192 kbps and bitrates above that, unless you've got a really nice sound system / headset. But if you do have a high-end system, you're going to want those higher bitrates, because the bass in particular is going to be so much better.
In any case, don't buy your mp3s from Itunes. Get them from Amazon (who offer higher-end variable bitrates) or the like. Less loss, no DRM.