Tim--
Now that your tracks are analyzed, the replay gain settings for album- and track-based can be changed at will.
Track-based is the classic apllication of RG. MC uses each track's analysis to set gain to the "norm" (which is still in dispute). I use this almost exclusively, as I hate volume 'suprises", and I almost never play original albums in track order.
Album-based RG will average the analysis of all tracks in a particular album and give each track the identical RG setting, which is a compromise. The reason to choose this is to preserve the original relative amplitude of the tracks in the album, as originally specified in the mix.
Alex B, for example, is a great fan of this, and has written at length about the artistic integrity of the mix, etc, etc, etc. But does any of this apply to the work of the famously impulsive Phil Spector, for example? I think not. I don't care what the audio enginer thought about the volume, I use track-based replay gain.