I've got a Killer NIC.

If you want the "best" system, its a very decent network card.
However, for the price, it is not worth it.
It does take a bit of the load off of the windows network stack, which might be marginally faster pings.
But the gain is very minimal.

The largest practical gain from this card, is that you can prioritize your gaming bandwidth, set your torrents/downloads to maximum speed in uTorrent (or whatever you use), and have the NIC give as much bandwidth to Blizzard as you can get, with everything left over going to downloads.
As in your downloads, even set at 100% speed, won't impact your gaming.

These cards are more for those with a lot of money.
Who want to be able to say, I have the best component in every slot.
On a budget, there are better places to spend your cash.