Whats the overclockable difference between 7800 GT and GTX?

Hello

Iv jsut been lookig at the two, and i can see a heatsink on the power thingy module thingys on the GTX's back while the GT doesnt, does that mean the GTX can yield a higher overclock? (thats inc upping everything voltage etc)

well, for the gtx's people around here have gotten 39x some even 4xx/and the high 13xx/low 14xx

in the reviews, the highest iv seen on a 7800gt to 395/1.22
but we all know they dont push the cards hard enough