If we stay with the Playstation, and compare the PS4 to the PS3, the difference is substantial.
PS3 was a beast when it came out. PS4? Not so much. Of course, Sony has stated that it does not want to be in the red for as long with this generation.
Why do I compare PS4 at launch to PS3? Because we could look at PS3 and equivalent PC graphics cards and see how long they lasted.
The PS3 GPU is slightly lower in performance than a GTX 780, at least according to the articles written about it at launch such as this one:
http://www.theinquirer.net/inquirer/news/1015532/playstation-gpu-slightly-powerful-geforce-7800
Let's just say that the 7800 couldn't have carried you very far. Optimizations is a very big deal. Even if the hardware is inferior you can get a LOT more perfomance out of it over time. Also, the major upgrade this time is primarily in the RAM area for the PS4. You rarely if ever need 8 gigabyte of RAM for PC games even now. Also, even if underclocked, 8 cores would actually be quite good if the developers optimized for it correctly, which is harder to do from an engineering standpoint(most games still rarely optimize for 4 cores even today, let alone 8, which is why Intel's processors are usually deemed better for gaming).
Also, as others have pointed out, there is no such thing as 'future proof', not even in the stagnating CPU world. If games actually start getting optimized for 8 cores, that could be an area where you'd want to upgrade quite heavily in 3-4 years.
I do agree with RussianSensation, however, that it makes more sense to buy a card for 300-350 dollars or so but upgrade more often.
For example, I bought a GTX 560 Ti 2 years ago as a midrange card. It has served me well so far and I'm in no hurry to upgrade. If I were to wait a few months I could get a 7970 for the same price as I paid, which would actually be lower due to inflation(and I'm assuming further price cuts as the new 9000-series gets rolled out). But I would essentially double my performance.
As I pay more than that, however, the return on investment diminishes significantly. Quite simply, you get less performance per dollar. So, it's better to save that extra cash, save some over the next 2 years or so and then upgrade again at the 300-350 dollar price range instead of getting a GTX 780.