The true potential of each card is un-knowable and essentially is only comparable via benchmarks which are largely synthetic.
Nvidia may have fewer cores, but those cores are capable of doing a lot more than a comparable AMD core.
The problem is that the graphics cards are not simply copies of each other made by different manufacturers, under the hood and well hidden by DirectX and the graphics card drivers they are in fact massively different beasts. Their entire architecture and way of doing things is different.
Nvidia is essentially similar to Intel, where performance and raw power reign supreme in the design ethic. AMD's designs tend to favour less "grunt" but more scalability so AMD compensate for lower power per unit by giving you many more units on their chips.
It is near to impossible to compare any Nvidia card to any given AMD/ATI card on the merit of how many cores they each have or the clock speed of each component.
The only real way to find out which one is better (at a given game or task) is by benchmarking it. A good comaprison site is VideocardBenchmark where you can find and compare most cards.