When you get iTunes Match, all music that is matched from your iTunes library is available at 256 AAC. That's an upgrade for your older 128 AAC tracks, but a downgrade from lossless tracks. Normally this will only affect the music on the iPad or other iOS devices, but it may be possible (so I understand) to sync these higher quality tracks back to your iTunes library.
The only mention I can remember of 320 MP3 was in a cloud music comparison. I think it was Amazon or Google that was using it for the songs your purchase through them.
Let me see if I can find that article. . .
Well, I can't find the exact article I remember, but there are a ton of them. If you are curious just do a Google search for 'cloud music comparison'; but set aside a couple hours if you are going to read them all (though I suspect the repetition after the first couple will discourage all but the hardiest of fact finders). It was Google who offers the 320 MP3 tracks, btw.