Thanks, King and Rocket. If I process all my mp3s with mp3s with mp3gain, I'll have to do audio analysis on them all over again, for the replay gain info MC wrote to the tags to be right, correct? But on the other hand, it seems as if the gain would be equalized so that I wouldn't have to use MC's DSP to read the MC replay gain data anyway?
MC's analysis uses 83dB as the target gain (per the original Replay Gain developer's specs) and MP3Gain defaults to 89dB. If you MP3Gain your files, it's not necessary to have Replay Gain enabled in MC.
Also, MP3Gain gives you an option of both track and album analysis. MC only does track analysis.
Also, MP3Gain has an option to undo the changes, but the undo info is saved in an APEv2 tag (instead of ID3 tag) which is non-standard for mp3s. It's possible (or likely) that some players will choke when they encounter mp3s with APEv2 tags on them.
If you decide to use MP3Gain with undo disabled, practice on some copies of your files first. I recall when I first joined this board, there were a few posts by people who had ruined thousands of files by applying incorrect gain settings.
Rob