
3.3V 64-bit PCI slot has the small segment first as well.
There shouldn't be any significant physical differences between the slots. The newer PCI cards - 3.3V will fit (and work) in a PCI-X slot, and PCI-X cards will fit in an older PCI slot (as long as the voltage - 3.3V - matches). The standards are backwards compatible with the slowest connected card determining the bus speed.