Go to SiS.com or ECS.tw.com and DL the latest driver for the onboard LAN if you have not done so.
And the answer to your question is nope.
Also, try disabling the onboard NIC and see if the problem persists. Then put in your PCI NIC and see if the problem persists. If the problem persists independent of a NIC, or with the PCI NIC installed, get a GeForce 3 or 4 and watch your problems go away.
Also make sure you have the latest SiS AGP driver, and play with your AGP aperture settings.
Since the "performance decrease" you indicate is completely vague, it is difficult to comment on what the problem might be. What I DO know is that you put in a new video card, and are using the onboard lan. Without knowing what you mean by "performance decrease," I would be pointing to the Video card before what NIC I was using; it is certainly the primary thing that has changed. If you mean lag, etc., then make sure you have the NIC driver installed, as I said above. Otherwise, put your PCI NIC back in and see if that helps.