I use The newest 20X liteon burner, ritek 1-8x discs (printable) at full speed in Nero 7 and out of the 10+ backups i have made and 30+ for xbox everyone has worked every time.
As for gamecube games i have never copied any, but if i try max speed they will work.
Quality burner + Quality discs = Quality backup
only SOME burners have to work at 4x