Winamp had a function, that I believe could do that.  I recall with winamp you could configure the equalizer based on the song that was being played, and save that setting.  With the amount of mp3s I have, that would be time consuming.  I would discourage you from any function that plays them all level, I don't think you will be happy.
Did you rip the songs yourself?  That is the best way to ensure that you are happy with the quality.  I have downloaded a lot of crap myself, so I ended up doing my own.
Good luck.  Post your results, we could all benefit from the knowledge.