This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
[From: Ralph Valentino (ralf@wpi.wpi.edu)]
In this case, the 64/128+ refers to the amount of data that can be handled by the video processor in one clock cycle. This affects the speed of operations on the video card, including transfers to and from video RAM and to the RAMDAC which generates the video signal.
This is completely independent of the width of the data bus which runs from the main CPU to the video card. As a result, a 192 bit video card can run on any of the buses available: ISA (16 bit), EISA & VLB (32 bit) or PCI (currently implemented as 32 bit - future 64 bit).
 
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