Hope these information may help.
Referring to the page 29 on the Technical Specifications Document from Intel Website, you will see from the diagram Figure 8: Server Block Diagram that:
The board has two onboard NIC GB Chipset 82573E and GB Chipset 82541PI.
One is travelling on a single dedicated PCI Express bus and nothing is shared with it.
But the other one is travelling on a PCI 32/33 Bus and it is shared with the Video ATI ES1000, so definitely, it will work on a bus speed of 32 Mhz and will slower.
Note: The Ethernet controller 82573E does not support TEAMING due to the fact that it supports AMT.
Refer to the page 48 for more details.