PC! card fits PCI-X slots?

[frankb]

I have a server board Intel SE7520, That I got at a computer show,
Building into a Game unit (I hope)

1 PCI- express x8
1 PCI- express x8 (actual x4 plumbing to a x8 physichal connector)

1 PCI-e regular.
and
3 PCI-X slots. Which resemble the PCI-e except an extra slot after the PCI to make it longer.
2 PCI-X 1.0 100 MHZ slots
1 PCI-X 2.0 133 MHZ SLOT

A PCI-e card that has two slots instead of 1 will fit, leaving nothing in the back end.

I have USB 2.0 card that is PCI-PCI-X havng two slots.
fits into both the PCI and the PCIx but the latter has extra slot free?

I take it to mean that the extra slot in back part is for Work-station card and/or a card that is PC/Mac compatible.

I would sure appreciate input to clarify.