Audio tracks with wide variation in RMS

this is called mastering.
 
 

The moment you are talking rms levels you should be also talking VU meters and ones that are real preferably with needles etc.  But there are some good alternate VST'as though such as the Klanghelm meter.
 
I usually end up doing all this fine tuning in a separate editing software like Adobe Audition etc.. Cool Edit Pro..
 
Firstly because I use the K system when I create an album all my tracks are at the same rms level but when there are variations eg from another pre mastered album this is what I do:
 
I open all the tracks up in an editor and then put a VU meter over the tracks one by one.  Now with a VU meter you have to give it a reference level.  eg a good mastering level might be -12 dB FS = 0dB VU.  So when the tracks are sitting around -12 dB rms wise the needle is just swinging nicely up to the 0 dB VU mark.
 
If a track is low it wall fall short. eg a -16 dB rms track will only make -4 on the meter now.  So what I do is hard limit the whole track so that no peaks go over -5 db.  You will find in a low level track like this that not many peaks are hitting that mark anyway.  Then I add 4 dB to the whole track.  The hard limiting prevents any odd peaks from preventing you from adding 4 dB to the whole track.  You certainly wont hear it.  Then the track comes up to a nice 12 dB FS rms level and the peaks are only hitting -1dB which is good for any conversions to mp3.  And now showing 0 dB VU like the others.
 
Loud tracks are going to swing the needle over.  So a loud track eg mastered to -10 dB FS rms will push the needle up to +2 dB VU most of the time.  So I just subtract 2 db of gain from the whole track.  Dont sweat any peaks now because you are lowering the tracks level.  Now the track will be at -12 dB FS rms and the needle should be just swinging nicely up to 0 dB VU like the others.
 
Do this and you will be going a long way to all your tracks being very even in volume and level all the way through. Burn a CD and check.
 
The only thing to watch out for is tracks that only have say guitar and vocal in them.  Even if you get them swinging nice up to the 0 dB VU mark you will find those tracks will sound a little loud compared to the band tracks.  I generally shave off another 2 dB off those types of tracks and then they sit nice against the band tracks.
 
Don't let anyone tell you that adding or subtrcating gain from final mixes is altering the quality.  That is rubbish and I defy anyone to hear the difference in an A/B test.  Remember Cool Edit or Audition is working in 32 bit/64 bit in the background which is often double the 16 bit resolution that the actual track is finally mixed at.  Even a 24 bit track being gain changed at 32 bit resolution won't create any issues either.
 
You need VU meters to do this.  Your peak metering is useless in this mode of operation.  And rms meters that are sitting way down on the scale are also useless.  You need something like the Klanghem meter that is swinging right up to the 0 db VU mark when the track is at the right level.  Then you really see what is going on.

Just put all your songs in one project.  Use Exclusive Solo to switch back and forth between your songs.  Pick your desired output level and either use the limiter and meter in Ozone to get each of your songs to be as close sonically and meter-wise to what you are shooting for, or use automation on the master bus to level match what you need.  
 
If you have ozone and sonar, you have what you need to get pretty close. 

My initial suggestions were based on all the mixes being completely mastered.  Only changing level.  Adding more dynamics processing to a final mastered mix is not a good idea.
 
But on re reading the OP I see if gentle limiting was used on these tracks only to prevent clipping then the tracks are basically unmastered I guess.
 
In that situation Bats is more correct here by saying mastering is actually required.  By that I mean you could in fact use EQ, compression and limiting.  All three of these stages can effectively alter the rms level of a track so then one could be mastering in full all the tracks and adjusting all the tracks that way.
 
One good bit of advice I learned from a great mastering engineer is group all of the tracks together that you intend to put on te CD.  Choose one track out of all of them first and master that one.  Choose a track that has everything in it.  Everything going for it.  Max instruments etc.. Biggest sound etc..  Choose a final reference mastered level and master the track to that level.
 
Then as you master all the rest have this mastered track available in your mastering for the other tracks so that you can switch any of the tracks against it.  It will keep levels and tone more consistent over all the other tracks.  Don't forget good reference tracks either as a comparison.

I had a friend who had similar issue and sent me 30 of his songs to take a look at. Noise reduction was one thing, but after that step I realized most of his material fell into one of two bins so made an Ozone preset to handle each scenario but I still had the issue of the variances in what he was sending me. The simplest solution for his material (also a "just to play in the car" type of situation) was to normalize each file to -3dB prior to the mastering pass with Ozone so that the Ozone settings required the least amount of tweaking for each. In hindsight I found the normalization pass effective for the quickest workflow. For his stuff, the workflow I ended up using was:

1. Noise Reduction and normalize to -3dB. Saved this to new file (destructive edit).
2. Added Ozone and loaded appropriate preset for the genre.
3. As the presets matured, I ended up spot-checking obvious portions of each song to adjust as needed (I used 4-bands for both presets).

I am not sure if the above will be applicable to you, but wanted to throw it out there. My friend's material had very consistent frequency content and dynamic ranges used, which made the situation a lot simpler for me.
 
This is definitely not a high detail/quality situation (as his material needed remixing anyway), but more a means to get consistent levels for the material he had.