My 9500's stock card was an ATI Mach 64 with 2 megs of Vram, how much does the Twin Turbo have? I'd guess this is the problem, not enough video RAM for OS X.

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cant be the VRAM i guess... the original iMac had only 2Megs as well and did work without a prb with OSX.

cant remember what card exactly was in there when i installed OSX. AFAIR it was some ATI card and i swapped it for a twin turbo afterwards which did work.