This video tutorial will help you quickly and easily convert your MP3 collection into the more space efficient and Open Source format known as Ogg Vorbis. This does involve a Lossy-to-Lossy conversion so its not 'optimal' and for some audiophiles they won't enjoy "losing" quality, but for the regular user, there's nothing to lose and everything to gain. (Including a nice amount of hard drive space)

http://winlame.sourceforge.net

Definition:

Phi, the Golden Ratio = (1+sqrt(5))/2
~ 1.6180339887

Since sound propagates through 3-dimensional space, it seems natural to me to say that the optimal quality to encode at is Phi^3 (less info)