320kps is the highest rate that mp3s can encode at (and as Tony said, higher than needed for most people.) Any VBR file (which the alt presets are) is going to use rates 320 and below depending on the complexity of the song. Alt preset Insane nearly is a 320CBR file because of the % of it's frames that end up using 320kps. Personally, I think that is over kill.

The only reason I can see recommending the external encoder is because there are other settings built into it besides bit rate such as frequency roll off etc.

FWIW, the guys that put together LAME use alt preset standard.

If your concern is archiving, I'd recommend ripping to FLAC for storing your files, then convert to mp3 Alt Preset Standard for your mp3 players or Ogg Vorbis, etc, whatver the flavor of the year is. FLAC is a lossless format great for archiving.