Ok I think I will get a used PRO/1000 PT on ebay and put in in the second x16 port.
The other card works too just by adding the pci id? Which kext is it anyways?
The Intel Gigabit CT Desktop adapter uses /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/Intel82574L.kext
Here is a guide on how to edit the kext to get the adapter working:
http://www.tonymacx86.com/network/3748-intel-gigabit-ct-desktop-adapter.html
The guide is for 10.6.4, but you edit the kext in exactly the same way in 10.8.3
Just for your info, the Genuine Mac Pro 4,1 and Mac Pro 5,1 have two Intel 82574L chipset Gigabit LAN ports which use the Intel82574L.kext listed above.
Dil83
P.S. The Intel PRO/1000 PT adapter uses /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/AppleIntel8254XEthernet.kext with absolutely zero modification needed. I have read that the Small-Tree driver enables WOL with this card, and it can be found here:
http://www.small-tree.com/Articles.asp?ID=198
I personally have not been able to get this driver to load after installation, the native Apple driver always loads instead so your YMMV.
The Genuine Mac Pro 3,1; 2,1; and 1,1 have two Intel Gigabit LAN ports that use AppleIntel8254XEthernet.kext
So by using either of these adapters, this will allow you to utilize the same hardware and native drivers used in previous or current Genuine Mac's. That's exactly what I was trying to do myself, so I own both of these cards.