Is it possible to put a PCI-X card in a PCI slot?
Solution 1:
Yes, as long as the PCI slot is a 2.x or later, PCI 1.0 was 5v while PCI 2.x was 3.3v - which is electrically compatible with PCI-X.
Source: Wikipedia PCI-X
Solution 2:
That isn't backwards compatibility. Putting a PCI card in a PCI-X port would be considered backwards compatibility - which is the case, as long as it is PCI 2 as the voltage on PCI 1 is not the same. To sum it up, PCI 2 cards can be put in PCI-X ports, not the other way around.