Very. Nvidia across the board have worse memory bandwidth than AMD cards in the same price bracket and at 1080p with no or little AA they usually trade blows, but not when you ramp up AA. Looking at the memory bus alone Nvidia uses 192bit for 660-660ti and 256bit for 670-680 while AMD uses 256bit for 7850-7870 and 384bit for 7950-7970. There are more factors to it (the last gen Nvidia 580 had a 384bit bus), but the proof is in the pudding as you can see below.
Here's a very good post about the 660-660ti and why the most common 2GB models aren't really 2GB. Kinda off topic, but it shows how memory bandwidth can be hid from consumers.
And I think from a technical perspective that Crysis 3 is really well optimized. They aren't the best around in making a pretty game art wise, but the beta used 100% of my GPU (common) and pretty much 100% of my four CPU cores (not common) so I think it moved tasks between both using as much of the total performance as it could. It's also the only game I know of where AMDs eight core CPU beat Intels quad cores. In raw performance AMDs eight cores aren't bad for the price, but making very threaded games is hard so most games performed much better on Intel's CPUs because they had much more powerful cores. It wasn't that uncommon to see even Intels dual core i3 beat eight core AMD CPUs.