Well, most gaming NIC cards generally state that it increases performance (in terms of ping) by drastic amounts. However, your ping most of the time, depends on the server you are connected to. You could have amazing download speeds, but have it regulated by the server you're downloading from. If you have a troubled internet connection, gaming NIC cards aren't for you -- because you could just call your ISP have it resolved.
And if you have a great internet connection, a gaming NIC card isn't for you -- because there's nothing to improve!
I've never believed the hype. I can see it making a difference on a LAN, but there are too many routers, servers, and what not sitting between you and the game server. You are only as fast as the weakest link in the chain.