You can actually increase the volume of your song files individually (I think that's what you're asking...) in iTunes. Just select "Get info" and under options you can change the volume to your heart's content!
What do you mean, something like the built-in volume adjuster in iTunes? (Mark all songs you want to gain, hit cmd-I (File -> Get Info) and crank up...) Edit: Beaten...
Just get an AirPort Express (and/or Monster Cable) and hook your iMac up to a decent hifi... then you can crank up the sound as high as you want without distorting the sound...
Use audio hijack to hijack your audio output stream and add a volume gain plug in. If that distorts then you really are already at the max headroom that the iMac's amp can pump out. If that's the case then your last ditch ditch effort is to add a compressor or multiband compressor plug in, but that will alter the sound of the song (radio stations all use compressors over the air). Or you could just go get your hearing checked.