I have an asus a8n sli, and it's working fine now with the new kext developed by eno, the one Voyn1x mentioned.
And having a look at your hardware it seems almost the same as mine, so I don't think that the problem is the lan controller.

This is my lspci:

00:00.0 Memory controller [0580]: nVidia Corporation CK804 Memory Controller [10de:005e] (rev a3)
00:01.0 ISA bridge [0601]: nVidia Corporation CK804 ISA Bridge [10de:0050] (rev f3)
00:01.1 SMBus [0c05]: nVidia Corporation CK804 SMBus [10de:0052] (rev a2)
00:02.0 USB Controller [0c03]: nVidia Corporation CK804 USB Controller [10de:005a] (rev a2)
00:02.1 USB Controller [0c03]: nVidia Corporation CK804 USB Controller [10de:005b] (rev a3)
00:04.0 Multimedia audio controller [0401]: nVidia Corporation CK804 AC'97 Audio Controller [10de:0059] (rev a2)
00:06.0 IDE interface [0101]: nVidia Corporation CK804 IDE [10de:0053] (rev f2)
00:09.0 PCI bridge [0604]: nVidia Corporation CK804 PCI Bridge [10de:005c] (rev f2)
00:0a.0 Bridge [0680]: nVidia Corporation CK804 Ethernet Controller [10de:0057] (rev f3)
00:0b.0 PCI bridge [0604]: nVidia Corporation CK804 PCIE Bridge [10de:005d] (rev f3)
00:0c.0 PCI bridge [0604]: nVidia Corporation CK804 PCIE Bridge [10de:005d] (rev f3)
00:0d.0 PCI bridge [0604]: nVidia Corporation CK804 PCIE Bridge [10de:005d] (rev f3)
00:0e.0 PCI bridge [0604]: nVidia Corporation CK804 PCIE Bridge [10de:005d] (rev a3)
01:00.0 VGA compatible controller [0300]: nVidia Corporation GeForce 8400 GS [10de:06e4] (rev a1)
05:07.0 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+ [10ec:8139] (rev 10)


If you wanna make sure it's supported by leopard, get a Realtek like mine, I've never needed a kext