it isn't Leopard - it is the graphics driver for the ATI card(s)
the X1600 is ok in windowed mode with XT12-08 in X 10.5.1 (but the normal maps are not working)
my G5 7800GT works without any problems, in 10.4.11
i do not know how other ATI cards or Nvidia cards work in intel macs..
imho is Apple losing sight of MacOS X, in favor of the iphone/ipod stuff and games are "ported" with this "cider emulation"