is there an app that can add replaygain values that uses ONLY tags (like eg vorbisgain)
so the original file remains untouched
i know it exists coz i used it but i no longer remember the name, could have been some player or plugin :)

Yeaaah, finally I see somebody from my country here.

But back to your question, I think Foobar is what you are looking for.

http://foobar2000.hydrogenaudio.org/

zdar :)
you know what, i think you might be right, i remember trying foobar some time ago :)

foobar can add a APEv2 tags to mp3 files with replaygain info (BOTH album and track gain and peaks, so you can switch from album to track mode whenever you want)