I'm not familiar with MP3Gain. Their setting of "89 dB" doesn't make sense to me. In Audacity, as in most (if not all) digital audio editors, 0 dB is maximum volume. Anything less is specified as a negative value, e.g. -3 dB.
If you are concerned that Normalize will adversely affect the stereo balance, then Amplify is the way to go. Just accept it's default settings and it will maximize the volume of the selection.
You might want to investigate "Chains": http://manual.audacityteam.org/index.ph ... dit_Chains
This allows you to batch process files. But Amplify is not the effect to use in Chains as the parameter that is retained is the amplification factor, which may necessarily be different for each song. Normalize is better in this respect as the retained parameter is the desired maximum level.
And as I said, the perceived "loudness" of a track does not necessarily relate to the maximum peak amplitude. http://en.wikipedia.org/wiki/Loudness_war