AMD is doing something similar that Intel did a long time ago. While Intel made the Megahertz Myth (that faster speed automatically means better performance), AMD is doing More Cores = More Performance. While this is true to a certain degree, for most consumer tasks, AMD performs worse than Intel. Also since the two companies take different approaches, you cannot do a direct comparison. To that end, the only way you can compare is with benchmark scores.
But here's something to put a dent that more cores automatically means more performance: http://www.anandtech.com/bench/Product/700?vs=677
How can a dual core processor keep up and even beat a quad core processor in some tests? Simple: AMD has poor instructions per clock (IPC) performance.
If you want to see a sadder tale: http://www.anandtech.com/bench/Product/188?vs=702
- The Core i5 I found here smokes the Athlon II X4.
Single core IPC performance is the underlying factor in how well a CPU performs. Which is something neither Intel or AMD will show you because, well, it's not rigid and it depends on what you're running.
And for one more trick, here's the Athlon II X4 against a Pentium G850 (dual-core), both sell for the same price: http://www.anandtech.com/bench/Product/188?vs=404
So yes, go find benchmark scores. Don't assume from raw specs that one processor is better than the other.