The Killer NIC is just marketing for suckers. $300? They know most extreme gamers know nothing about networking and just go out and buy the fastest, most expensive gear they see in ads. The only instance it may be worth it is if you have it in a LAN server for local matches. In which case it's dedicated anyway and you wouldn't have it also downloading music and email during gaming. The Killer NIC can't do anything about incoming packets from your ISP. They arrive in whatever order they arrive in. A better performance increase can be found in adding RAM, faster video, or faster disk.
As for a NIC card vs. onboard, if you have a CPU faster than 2.0Ghz it isn't going to be the least bit stressed by the network interface. The onboard has a controller chip and driver, just as the PCI NIC card. And the new Gig onboard ports are still limited by your 10/100 router, which is further limited by the 6MB ISP connection.
So use the onboard if you have one, otherwise any brand name $35 ethernet adapter will give you all the performance you'll ever need at home.