I'm not sure about the overclocking part, you may be referring to another model that they also offer. The two models shown differ from what I see and find on the web site in the cooler that is attached. see more here
http://www.theinquirer.net/Default.aspx?article=36719
or here
http://www.theinquirer.net/default.aspx?article=35604
or to save some time
"Almost three months later EVGA has announced the card with this cooler. But the new overclocking bios makes the card work at 626MHz core and 2.0GHz memory. The standard bios makes the card works at 575MHz core and 1.8GHz memory. A second card based on ACS3 is 8800GTS clocked at 580MHz core and 1.7GHz memory.
The cards are about to become available with etailers with suggested retail price of $649.99 for the 8800GTX KO ACS3, and $479.99 for the 8800GTS KO ACS3.
We already wrote that ACS3 is more efficient than Nvidia's own cooler and can keep the temperature down a few degrees more.
In addition to the new ACS3 models, EVGA is also announcing two Superclocked models, with Nvidia reference cooling. The faster one is called e-GeForce 8800 GTX Superclocked (P/N: 768-P2-N835-AR) and works at 621MHz core clock with 2.0GHz memory while the second one e-GeForce 8800 GTS Superclocked (P/N: 640-P2-N825-AR ) is clockerd at 576MHz core clock with 1.7GHz memory. "
The 51mzh difference is mentioned, I just wasn't sure if they meant it came that way or that with the bios you could overclock it yourself to that, it does seem to indicate it comes that way out of the box though.
hope this helps.