Save yourself a huge load of hassle and go get a couple of these, and then do your upgrade. Hopefully your PCIe x1 slots aren't already being used:
http://www.newegg.com/Product/Product.aspx?Item=N82E16833106036
I personally stay far away from Realtek NIC controllers on my VMware whiteboxes. The day I made that decision, life got a whole lot simpler.
The Realtek controller on your motherboard doesn't work with ESXi 5.1. VMware changed the Realtek driver on that release for some reason, which made that controller on your motherboard stop working at 5.1.