Peripheral Component Interconnect(PCI) was the first system used to transfer graphics data. It sent graphics information through the main bus that also relayed most other information on the computer. This, therefore, slowed the transfer of graphics data.
In 1996 Intel introduced Advanced Graphics Port(AGP). This provides point to point data transfer that does not have to work around other information. AGP, unlike PCI, sends all the data needed to render an image at once and in one file. It also addresses the packet of data so that your computer doesn't need to open the packet to find where is must be sent. This in turn is a much more efficient way to send graphics data.
In 2002, PCI-Express was accepted as a new standard in data transfer. Instead of one pipeline through which all data is tranferred, PCI-Express features multiple 'lanes' interconnecting each device individually. Though the clock speed of PCI-Express is already significantly higher than that of the previous PCI standard, the main increase in performance comes from this system of multiple pipelines.
There are numerous factors to video quality. These include a video card's ability in regards texture filtering, handling pixel shaders, as well as things such as color depth, maximum resolution, and the actual clock speed of the video card.