The efficiency hasn't improved that much in 5 years.
Clock speed is still the number 1 concern.
More cores is secondary to clock speed. More than 2 cores aren't taken advantage of often at all.
I still use my dual core intel 6300 overclocked to like 2.1ghz from a few years ago (2006? 2007?), and it runs everything at an okay resolution. Mostly the graphics is the real bottleneck in games. Most benchmarks you see of CPUs kicking another's ass is with all the graphics options turned off. Once you turn on the options most people are definitely going to want on at the very least, things level out a whole lot more because the graphics card becomes the bottleneck.
I'm thinking of upgrading, but really, there's not that much reason to yet. You don't get every single bell and whistle with everything cranked up, but i don't even notice it unless you're talking about some brand new hardcore FPS game and you can't crank the draw distance all the way up in a wide open area, and even that usually doesn't matter too much.
It's never even occurred to me anymore than games have system requirements. Everything just runs. I've run mass effect 2, bioshock 2, dragon age, and newish games like those just fine. I played through Psychonauts at 1920x1080.
