The X5DL8-GG has two CIOB-X2 bridge, each containing two buses.
First CIOB-X2:
S1 Shared between PCI-X Slot#1 and the Onboard U320 (Adaptec 7902) SCSI - the maximum speed is 100MHz
P1 Shared between PCI-X Slot#2 and Slot#3 - Use one of these PCI-X slots and don't populate the other slot. If both slots are used the maximum bus speed will be 100MHz.
Second CIOB-X2:
S2 Shared between PCI-X Slot#4 and Slot#5 - Use one of these PCI-X slots and don't populate the other slot. If both slots are used the maximum bus speed will be 100MHz.
P2 PCI-X Slot#6, Broadcom 5703CKHB - if onboard Gigabit disabled the maximum speed is 133MHz, otherwise both will run at 100MHz.