320kbps VBR MP3 doesn't make any sense. 320kbps is the maximum bitrate for mp3, so it would have to be CBR. A 4 minute song at that rate would be 9.5MB.

I don't think WAV makes any sense on the empeg. It'll keep your hard drive spun up all the time and you won't get any better quality than FLAC. A 4-minute 16-bit 44.1kHz stereo WAV file will be 40MB. Since it's the largest possible file for that data, it's the only thing that even fits your highest category.

FLACs average out to maybe two-thirds to four-fifths the size of an equivalent WAV, you don't have a category it will even fit into.

I can't think of any format that would fit into the 12-15MB category.

You could have arguments about which of Ogg Vorbis, mp3, or WMA sounds better, but they're likely to be subjective. However, assuming that specifying the bitrate for your VBR mp3s means you're doing the average bitrate thing, you'll get better results by going with a quality preset that doesn't necessarily equate to a specific bitrate.

Personally, I just go with lame's --preset standard mp3s for virtually everything.