The current generation of quad port cards are more than meets the eye. They have a bridge (or switch) chip which creates a secondary bus that can make for trouble when debugging for performance. The driver can only report what the MAC reports, and MAC always runs at maximum speed since it is on a private bus. But the card, and therefore the connector chip (bridge/switch) might be in a slot that doesn't yield the maximum speed. While it is called a switch in PCI-E land and a bridge in PCI-X land, I’ll call it a switch from here on out to keep things a little more clear.
Here is what it can look like in block diagram art (Host is below the Green Arrow):
The two red arrows are the backside bus and they will always run at highest performance allowed by the connection. The light green arrow is the front side bus and it is at the mercy of the slot. Because the switch is in pass-through mode, it’s hard to see it running at the wrong speed. Here's what you can do.
Make sure when you are physically inserting the card that it isn't a slot that is different between the physical and the electrical abilities. A ton of motherboards today will have just one physical connector type (typically x8) and vary the electrical (x1, x4, x8) depending on the board. Consult the documentation or inspect the motherboard, most boards these days are clearly marked.
This Back side / Front side problem gets worse if you’re still using our older PCI-X adapter cards. It’s easier to exceed the front side capabilities when each backside channel is running at 133Mhtz speed. PCI Express will typically have less of problem with this since its just plain old faster, and really enables some great networking speeds. But that’s a whole ‘nother post.
Let’s go out with the big finish:
1) Be aware of Front side / Back side speed and protocol differences and maximize for both
2) PCI Express has more flexibility Front side / Back side than PCI-X
3) Thanks for using Intel networking products.