Well, most peoples PCI system works at 33Mhz (unless overclocked) and AGP works at 66, therefore it's quicker than PCI. An AGP card is allways a little quicker than a PCI one, at least they are in the benches I have seen.
AGP 1x works at 66Mhz, 2x at 133 and 4x at 266, but to be honist the x number doesn't make that much differnce performance wise, because not much uses the AGP transfer system.
PCI cards react better to overclocking. As 1040stf said, when you overclock your FSB it also overclocks the AGPbus. it overclocks the PCI bus too, but AGP runs at PCI clock x2, so the incrase is x2 as well. So if you are overclocked to 133MHz FSB, your AGP bus has gone from 66MHz to 89MHz. Now, if you're running an AGP 2x grafix board, it's gone from 133Mhz normal speed to 178Mhz. Thats starting to get a bit excessive, whereas a PCI card in the same system would be at only 44.5MHz, much closer to specs, less likely to yell.
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AHHH, I think my computer's got a Virus!
Oh no, that's Windows 98