Ok I think I'm getting it now.
So they all basically use the same parts/specs/chipsets/etc... but certain companies allow differing levels of overclock-ability?
they using same chips, same design, but electronic components, memory chips, quality of pcb can be different
my saphire went easily to 900/1250 @ 1.11v and can most likely be pushed even further (690 khashes so far
if you want to overclock get yourself decent aftermarket cooler - http://www.arctic.ac/en/p/cooling/vga/19/accelero-xtreme-5970.html?c=2182
keeps my overclocked 5970 under 65 degrees celsius and is nearly inaudible