This problem is pretty common when using virtual machines and you clone them.
With CentOS 6 everything is handled by udev now.
Go into /etc/udev/rules.d and delete the 70-persistent-net.rules file and reboot.
If you open it before-hand you will most likey see the original NIC MAC listed as eth0 and the new one as eth1.
Now you need to edit /etc/sysconfig/network-scripts/ifcfg-eth0 and manually update to the MAC of your new NIC card.
Deleting the file forces the detection process to run again at boot with no baggage left over from the cloning process, namely the old NIC MAC address(es).