No pci-e comes in x1, x2 ,x4 ,x8 ,x12 and x16 types.
you have 2 choices, if your x8 slot has an open end it will physically accept a pci-e x16 card but work at x8 speed (still much faster than pci or agp) or get a x8 graphics card wich may be hard to find but i do believe theyre about.
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