The reason M games usually do badly on the GC, in my opinion, is that if they come out they're six months behind the other versions, shitty ports, or both. Yeah, Cube owners are really going to fork out for that crap. Kind of bad example: Soul Calibur II outsold the other two console versions. Why? Didn't suck, released day and date with the others, had Link, so GC owners bought it. Pretty crazy, though, that it outsold the version for the PS2, a console with a much larger install base.
Eternal Darkness didn't do so well, but the RE games seem to do okay. I mean, they keep makin' them. The RE 1 remake kind of stole some of ED's thunder, in my opinion. Haven't seen anything on sales of Metal Gear, though.
That's enough inane rambling for now, I think.