> Quad G5 [...] won't be really so much better performing than currently supported
> top-of-the line G5 (except for the gfx speed of course).

I think the main advantage of the Quad G5 (and other PowerMac11,2 systems) over currently supported G5 systems is clearly the ability to use rather modern, easily obtainable PCIe graphics cards (even non-Mac ones with some restrictions).