the differnece is in the cooling. they all of eVGA's oc'd cards have about the same specs, but the cooling solutions are different.
the gs has 400 mhz fan or passive cooling options
the gt has 560 mhz and "With a copper thermal solution."
the co has 580 mhz and "With a copper thermal solution."
the ko has 580 mhz and "With EVGA's own high-efficiency thermal solution."
more specific answer...
EVGA KO Edition Video Cards
The new ACS³ design not only derives itself from the original asymmetrical fan design, but combines an eye to the cosmetic, large heat sinks like found in the earlier designs, heat pipes, and new materials technology to dissipate heat at a whole new level. With the release of the 7800 line of cards, and the market's requirement for smaller cooling solutions, it is now the perfect time to unveil the new ACS³. Based on our ACS³ technology, our e-GeForce 7800 GTX KO Edition is the only card in the marketplace that can operate at 490+/1300+ with real stability and confidence.
With EVGA's patented ACS³ cooling technology mated to one of our new 7800 GTX KO Edition cards, you can lower your core GPU temperatures by about 5°C to 7°C, and your memory temperatures by about 8°C to 10°C - creating an environment that is much better suited to high performance clocking.
and the co...
EVGA CO Edition Video Cards
Another addition to EVGA's new EVTweak initiative, the CO Series with Copper Heatsink is the next installment of the company's innovative leap forward in VGA cooling design, delivering unparalleled horsepower harnessed under the hood of EVGA's custom and stylish all-Copper heat-sink fan.
and as far as the superclocking...
EVGA SUPERCLOCKED™ Video Cards
The EVGA SUPERCLOCKED™ series of cards are a culmination of thousands of hours of torturous testing by our Lab Engineers to ensure that you get the absolute best bang for your buck. EVGA SUPERCLOCKED™ cards exemplify the high quality that you have come to expect from EVGA, right out of the box.
Born and bred from the EVTweak™ Initiative, SUPERCLOCKED™ is a pedigree that has proven itself time and time again to be second to none with the likes of CO SE, CO and the KO Editions of graphics cards that are unique to EVGA.