It's not necessarily just the question of number of cores. The actual architecture and chip design matters a lot as well. A Pentium 433 clocked to 4.0 GHz won't outperform an i7, for example.
The family and market position of a card is far more effective at determining power than actual core numbers and whatnot, for most people. The 690 is better than the 680 which is better than the 580 which is better than the 480.