It hard to compare graphics cards in a AMD vs Nvidia manner. You can use Specs as a useful guide when comparing two GPU from the same architecture ie: R9 270X vs Radeon HD 7870 as they both use GCN. note the r9 series is very slightly tweaked version of GCN, ie GCN 1.1. the GTX 660 and HD 7870 were in similar performance and price range last year. in terms of GPU compute power victorst1 has it right in that order. Just for a point of reference the HD 7870 has about 2.56 TFLOPs and the GTX 660 has about 1.9 TFLOPs. But there is a good amount of AAA titles that teh GTX 660 out performs the HD 7870. Also Video Memory can perform differently on the same Architecture for example a GTX 770 2Gb on a 256bit bus clocked 7010MHz can better offer a bigger buffer than the exact same GPU in the GTX 680.