Thats it. Best way to NORMALIZE files, when you like them to be really loud?

Can be normalized to 100 % ?
Whats the best way to avoid clipping?
What the hell is RNS and why in Sound Forge the peak level of a file is always 0.0 ???

if you're talking about mp3's I suggest you to use mp3Gain + the max gain feature

Maybe you want them to be rather quiet in future...
In Cool Edit I normalize to -0.01dB to surely avoid clipping. In any case, manual control is necessary.
Do you mean RMS ?
RMS=root mean square
I think the related formula for loudness calculation is
1/(t[2]-t[1])*sqrt(Int((f(t))^2,t=t[1]..t[2])), where f(t) is the varying loudness, the curve which a wav editor displays, and t is the horizontal time axis.
(I hope you know maple or mathematica...)
RMS volume is a way to measure loudness close to the way humans hear volume.

The peak volume is always at 0dB because the CDs' peak level is always 0dB. This is sensible because they're trying to make CDs as loud as possible.

How I do that? What I must use in the Target "Normal" Volume textboxes ? [Im using the GUI]

I think you have to turn on this option first...so go to: Options > Advanced... and click on "Enable Maximizing features". After that you can use them clicking on your file(s) with the right mouse button