The relative level of songs is something I worry about. I'd set one track as a "reference" and compare all the other to that, otherwise you will tend to creep louder or softer as time goes on.

I have wondered whether to record all the songs using a PC (at a low bit rate like 8khz to save space; also most of the loudness is at lower frequencies) then examining the waveform (using Soundforge).

Trouble is peak amplitude is not a very good measure of loudness which is more a question of "density". If you look at a professionally mastered track it is astonishing how crushed together it looks. I think the UK magazine "Sound Technology" pointed this out for the U2 song "Elevation".

So for now I'm trusting my ears!