Because very few mp3 players support ReplayGain, here is what I suggest.
If you have enough hard drive space, copy your files to a separate directory. Then run the MP3Gain program which can be downloaded from
http://mp3gain.sourceforge.net/download.phpI use a level of 92 to avoid the volume level being too low on the portable. Transfer your files to the portable, and they will play at similar volume.
If you are constantly moving files on and off the portable, this is not a good solution. But if you have a high capacity portable, say 20gb or more, then it works better because the first mass transfer will take care of most of your music. Periodically, you can add new music after copying and applying MP3Gain, then transferring those tracks.
The big advantage is that you are not limited to using only an iPod. Any player that supports MP3 (which is everything) will work. Therefore, you can consider players from Creative and Dell and others which can save you money over an iPod.