Given two ethernet cards as follows
When both ethernet cards share the same 1GBPS rating, how are the additional 32-bits relevant? EDIT: Attached link to a few I located on ebay
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A 64 bit PCI card will fit in a PCI-X slot (not to be confused with a PCI-e slot). In practice, standard PCI-33 has enough bandwidth to feed a 1 gigabit card, so you're unlikely to notice much difference in performance, even if you put the 32 bit card in a 64 bit slot. Note that the slots and cards are compatible both ways.
PCI-X slots are usually seen only in servers, desktop machines ususally only carry only PCI and PCI Express x1 slots, although high-end workstation systems often have PCI-X and wider (x4 or x8) non-video PCI-e slots.
However, the bus speed will make a difference. If you have a PCI-33 card in a PCI-X/100 slot, the 33 MHz card will slow that whole bus down to 33 MHz. This could make a difference to other cards on that bus. A 64 bit PCI-X/100 slot has a nominal bandwidth of around 800MB/sec. If you put a 33MHz card in this slot it would slow the whole bus down, reducing the available bandwidth to 266MB/sec. If another card on the bus could potentially transfer data faster than that (a RAID controller, for example) then its available bandwidth would be constrained to 266MB/sec.
I have seen this happen in the wild. If the machine has more than one PCI bus then you can put the slow cards on one bus and the fast cards on another. Most servers and older workstations with 64 bit PCI busses have more than one bus, although 1U or 2U servers may only have slots available on one bus.
Simply that data can be exchanged between memory and the ethernet cards in 64bit lumps instead of 32bit ones, ie 8 bytes per fetch or put instead of 4.
Therefore 64 bit cards need about half as much processor time or bus utilization than 32 bit ones, for the same transfer rate.
They will also need different drivers aware of the differences.