Unlike 32-bit PCI buses, which can be found in all current x86 computers, 64-bit PCI buses are still a prerogative of servers and professional workstations. The bottleneck of a gigabit network adapter operating on a 32bit bus is the bus. In case of 64bit, it's the adapter. In fact, the CSA bus from Intel (reviewed below) solves the "bottleneck" problem of the bus with gigabit network adapters installed on common computers, but not all mainboards have CSA and it is used only for integrated adapters.



Link: Digit-Life