64 bit PCI is 64 bits "wide" (i.e. it sends 64 bits in parallel at the standard 33mhz pci bus speed). Much like Serial ata, PCI express is serial (i.e. it sends 1 bit at a time) this may sound slower, but there is signifigant overhead with going wide. Because different wires are carrying different bits, the bits may arrive in the wrong order, or slower than each other. Because of this, there is significant overhead in "wide" busses which make them untenable at higher speeds. Since serial sends 1 bit at a time, it can go much faster, because no one worries about bits arriving out of order. And if you want more speed, you just use multiple 1 bit lanes, instead of a 1 "wide" lane. 64 bit PCI is a stop-gap measure to improve the speed of the PCI bus for servers, much like AGP was a stop-gap measure to provide more bandwidth to graphics cards. PCI express will get rid of the AGP bus with the 16X PCI slot (yep, 16 1-bit lanes, running at super high clock), and also go faster than 64 bit PCI.
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