Ok, let's do things in order:
- Instead of Rammfire (which I've tried, as well as every other amp sim there is

), try using some of LePou's plugins, especially "Legion" model - it has the cleanest and most defined distortion. I still consider it to be the best sounding amp sim out there, and it's free!!. Then Insert any IR loader (LePou's also has that too), and load some decent IR, there are tons of them here on this forum. Personally I prefer GuitarHack's impulses.
- As far as the mastering goes - If you're still not quite good with the mixing, then don't venture into mastering yet. If your mixes are properly done then mastering shouldn't be a complicated process. Personally, once I sort out the mix I never have a problem with achieving the loudness during mastering which tells me that the mix is finally balanced. If distortion or clipping starts to appear way before you've achieved the desired "radio" level, then your mix is still not good, and you shouldn't try to fix that during the mastering, but in the mix itself. For now, just consider mastering as a process of achieving the loudness level (although, of course, it's not just that), but that will tell you if your mix is good or not.
Good luck