overclock it to what? At this level they are both the fastest single cards, and you are worried which one oc's better? makes no sense.
If you wanted to get the gt/gs (or ati equiv) version of a given card and wondered if it would reach a level equal to the gtx/xtx version then I see the logic, but at this level you are saying "how high can i get" w/o having a unit of measure.
I could say "you can get up to 14billion pico-pajixels" but you have no frame of reference, so it means nothing.
If one oc's 50Mhz and the other 25, they are such radically different architectures that the 25 may still outperform the 50, or you could see no gain at all in a real game. Plus there is no certainty that YOU would achieve the same oc.
You need to buy based on the architecture, which is by nature based on what games you play... once you figure that out, and you have the $ to get the top end one for said arch, just get it and be secure knowing that it is the fastest.
sorry, end of rant. :roll: