>I've heard that there is a good GameBoy emulator working on Atari ST >computers. Is that true?
There is one in development (STemboy). It doesn't run commercial games but the manual says that some homebrew ones work fine. At least Super Mario shows the title screen.
>I also want to know where to find a RSC-editor like Interface 2? Is there still >someone supporting it (or another prog like that)?
Resource Master by Application Systems. It's still supported.