first of all dont bother with overclockers... it is wayyy more expensive than other uk online retailers
or compare prices at:
Anyway onto your question:
diferent card manufacturers clock their cards differently... some have a gpu core speed of 400mhz and others are set at 500mhz
they also change the memory clock (mine is set at 1000mhz, though others are set at 500mhz)
so yes there are differences between 6600GTs
Gigabyte for example make 2 pci-express 6600GTs one with no overclocking (set at something like 400mhz core/500mhz memory) then they have the turboforce edition (which I have) which comes at 500mhz core/1000mhz memory).
so there are differences and some are better than others.. it pays to look at the specs of cards