I have 500 mp3's in one folder on my harddrive. I want the volume to be the same on all of them so I don't have to use the volume control everytime a new song starts.

If I use mp3gain and do a "track analysis" on the whole album and then apply "track gain", will all mp3's then be dynamically compressed?

No, mp3Gain only changes the volume levels of the mp3 files, more specific, change the volume levels of every frame of the m3 files with the same value, does not apply dynamic compression at all, I suggest use the default value of 89 db, the files maybe sounds more quiet but at the same volume and without clips

Each mp3 will have the same volume to your ears, yes.
"Dynamically compressed", no.

-Glen