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Video Card "Bit Width" vs. System Bus Width
It is very common for video cards to be referred to via a bit size. For example, a video card will claim to be 32 bits, 64 bits, or even 128 bits in size! It is important to realize what this means. It refers only to how much data the video card can handle internally. It refers to the width of the bus between the video processor and the video memory, usually. When interfacing to the system processor, the video card uses the width of whatever bus it is connected to. This means 16 bits for ISA cards, 32 bits for VLB, PCI and AGP.
While a video card that is 128 bits seems like it would be twice the speed of one that is 64 bits (and the video card makers who produce these cards don't do much to discourage this impression), in fact this bit width is only one factor in overall performance. See the performance section for more on this. At any rate, you can use a 32, 64 or 128 bit video card on any system bus that it will plug into (you don't need a 128 bit system bus, which is good, because they don't exist!)