By the naming conventions, the EVGA 55nm cards are numbered 896-P3-125x, and the 65nm cards are numbered 896-P3-126x. Also, the 65nm cards have a more orange casing, whereas the 55nm cards have a red casing. Have a look at the pics:1255 GTX 260
and 1265 GTX 260
Note, these are the Core 216 editions of the card, I left out the Core 192 versions, since they're all 65nm, and really aren't worth the $20 savings. YMMV, but I bought the stock speed 55nm card and haven't had any problem overclocking to the Superclocked edition speeds.