P.S.: Operating System is Ubuntu 12.04 LTS Server & both NIC's are detected upon startup as is confirmed with the console command: lspci
However, running the console command ifconfig after a fresh startup shows that when the Realtek NIC is physically inserted into the motherboard that the onboard Intel NIC shuts down and does not send or receive any packets at all.
It is clear that there is some kind of interaction going on with these two cards and because the BIOS is so restrictive my hands are tied, I cannot do anything.
Oh, also the CPU is a Core i3 2120-T and the RAM is 1x stick of 4GB Corsair.
Hard drives are two Western Digital Caviar Green's 2TB configured in RAID1 by the onboard Intel RAID controller chip.
Changing PCI Legacy setting from 32 to 128 has no effect.