33MHz 64b PCI is just that: 33MHz 64b PCI. PCI-X is both faster and 3.3V only. Not that the distinction PCI vs PCI-X is too relevant, it works exactly as any other PCI bus would at those speeds.

Right, the long 64-bit connector - that everybody just calls the "PCI-X connector" - in fact long predates the PCI-X specification (1998), and instead goes back to PCI 1.0 in 1992.

When did computers start to appear that used it?

I see that there is such a thing as 32-bit 66 MHz PCI as well. Has anyone seen one of these slots?

Maybe not quite what you're thinking of, but the Intel L440GX+ server board has two 32 bit 66MHz PCI slots (5V) bridged off the AGP interface.

Errius wrote:

Right, the long 64-bit connector - that everybody just calls the "PCI-X connector" - in fact long predates the PCI-X specification (1998), and instead goes back to PCI 1.0 in 1992.

When did computers start to appear that used it?

Iirc some more high-end Pentium Pro chipsets, so ~1996, although some Alpha systems also started using them around this time.

I see that there is such a thing as 32-bit 66 MHz PCI as well. Has anyone seen one of these slots?

Check the last pre-AGP Apple systems.

Errius wrote:

I see that there is such a thing as 32-bit 66 MHz PCI as well. Has anyone seen one of these slots?

IIRC the nForce2 chipset supported 66 MHz 32-bit slots, because I remember hunting down a 32-bit 66 MHz capable gigabit ethernet card for it, and was very happy when Linux reported the card running at 66 MHz. There are usually some restrictions, such as a few slots share the same bus, so inserting a 33 MHz card will force all cards on the bus to drop back to 33 MHz (even those that support 66 MHz) but for some reason I don't remember having this issue with the nForce2 board.

What Windows program can you use to see the speed individual cards are running at?

No idea but you can always boot an old 32-bit Linux live CD and run "lscpi -v" to see 😀