There's not really any such thing. 320kbps is the highest bitrate that mp3 will do. So in order for all the frames to average to 320, they'd all have to be 320.
In all honesty, I'd do a blind comparison. Find the one with the lowest bitrate that sounds no worse than any higher bitrate, then use the next higher step. Personally, I use lame's --preset average, which averages around 190kbps, which is a hell of a lot smaller than 320.
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