You don't need a hungry dGPU for a server like this. A COD4 server can work just fine with an integrated GPU. 8GB of RAM might be a bit anemic, considering you want other game servers running as well. Of course, it greatly depends on the game servers you want to host. A Minecraft server for instance should really have 2-3GB just for itself to run smoothly, and it'll take one or two CPU cores as well.
I'd stay away from the Xeons mentioned in the video at the start of your post, as they're Core 2 based and very power hungry (130W TDP for the quad cores). Ideally you'd want a 1366 board and 6-core Xeon but that's out of your price range.
For this price range, I'd suggest a socket 1156 board with 4 RAM slots (H55 and P55 chipset are both fine) and a Xeon X3440 and fill it up with RAM. To reduce power consumption, throw in a GT210 or some other basic PCIe card you have.
