PCI boards can be keyed in 3 basic forms: 3.3V, 5V, or Universal. Keying refers to the shape of the connector where the PCI card connects to the PCI bus. The 3.3V and 5V keying only supports PCI buses with their respective voltage. Universal keying supports both 3.3V and 5V PCI buses. Each of the above keying types also comes in a 64-bit and 32-bit form. This gives us a total of 6 possible combinations. The following diagrams show the various types of PCI board keying and each board's compatibility with different PCI bus architectures:
The PCI specifications allow for both 5V and 3.3V logic levels used to communicate between the PCI bus and the board. It has nothing to do with what voltages are supplied to the card for powering purposes. Even when used in a 3.3V PCI slot, PCI boards will consume the same power from the 5V power rail.