Both-ways compatibility, but the lowest common denominator wins.
It gets worse if there are onboard PCI-X presenting peripherals. Where the contention bus (the E7505 chipset has 3 PCI/PCI-X buses over however many sockets plus AGPPro 8x) has a mixture, it is a pity to drag a PCI-X 66,100 or 133 bus down to PCI32 33MHz.
What you ask is normally fine. There might be some exotic PCI-X cards that don't plug into PCI, but SCSI and LAN adapters are almost cetainly fine in 32bit slots.
Its the G5 that's bought all this on. I don't know if it's something about the way the G5 runs PCI-X or if it's that a lot of these cards don't conform to the full spec. I do know it's a 5V versus 3V issue.