That particular Ethernet controller is problematic to say the least. There are some ways to make it work,but from what I have read unless you are well versed in kext editing you may be out of luck. Your best bet is to buy a compatible NIC for your system, could save you a great deal of grief.
There are some folks on here that may be able to help you along in case you are determined to get your on board NIC to work. Below is a link that I found just by accident. Read up on the issue and maybe you can fix it. Good luck.

http://www.insanelymac.com/forum/lof...40851-500.html

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OS : Snow Leopard 10.6.8
Motherboard : Gigabyte GA-M68MT-S2
CPU : AMD Athlon II X4 645, 3.11 GHz
RAM : 4GB Crucial PC3/16000 DDR3
GPU : nVidia Geforce 210 1 Gig Full Hardware Acceleration
Sound : Onboard Audio Realtek ALC887
LAN: Sonnet Gigabit PCIe Ethernet Controller
Monitor : Acer S231HL 24" Wide Screen 1920 x 1080 @ 60 Htz
USB: Keyboard/Microsoft Confort Curve 2000 - Mouse/Logitech M185
Bluetooth: Apple Magic Tracpad/ DLink DBT 122 Bluetooth Dongle