http://forum.shmups.com/forum/viewtopic.php?t=87
Oops! Posted this before noticing your location. The information below is for NTSC (60Hz) RGB.
Playstation, Playstation2, Xbox, Saturn, and Dreamcast can output an RGB signal. I understand that the Master System, Genesis 1, Genesis 2, and 32X can also output an RGB signal, but I don't have first-hand experience with them.
For the North American Gamecube, it is not possible to output an RGB or VGA signal out of the box. Gamecubes made after
May 2004 lack the Digital A/V Output port. If you have this port, you will need to modify a Gamecube Component cable or Gamecube Digital Video cable (D-Terminal) to use either RGB or VGA.
note: I haven't performed this cable operation
I think I read that Europeans get RGB support fine from their Gamecubes; Nintendo gave them the shaft in other ways.
PC-Engines,
New-style SNES, and
Nintendo64s need an internal modification to output RGB. GamesX is not necessarily the be all, end all of modification knowledge. I provide these links so you can get an idea of what's going on. If you're serious about performing the mod, try to find some other guides and compare notes.
note: nor have I performed these operations
NES/Famicom is stuck in RF or possibly composite.
I do not know for certain the situation of the original SNES or the Genesis 3.