RE: comparison with a PC and PC performance
Cross porting may not be as easy as all that.
At present, every PC game relies on high level API's to inter face with the underlying hardware - DirectX, OpenGL for 3D rendering. These calls are pretty damn inefficient at exposing the true power of the hardware. This is by necessity - the same API's have to abstract a huge range of physical GPU's from a variety of manufacturers, so this loss of optimisation is to be expected.
You've then got the operating system layer, which operates as a go-between from the code to the hardware, again abstracting to cope with a wide range of hardware varients.
If everyone wrote they're PC games in x86 Assembler, and their graphics code in the AMD or Nvidia equivalent, we'd see performance an order of magnitude better than we do now. Of course, that's not realistic as that code wouldn't be portable to the near infinite number of hardware configuration varients found in the PC world, and I doubt x86 assembly is much fun to work in these days...
Even if the PS4 (and next Xbox) use x86 based CPU's and a variant of the Radeon GPU, they're going to be a single fixed part for the lifetime of the console - Meaning Sony can provide bespoke API's that are much more closely coupled to the hardware, or even provide direct access to the hardware for particularly performance focussed developers to tinker and squeeze out the maximum performance with - Thus the "real world" performance of a game on the PS4 is going to be extremely good compared to its PC version running on basically the same hardware, which is having much of its performance sapped by unavoidable inefficiencies at the API, driver and OS layers.
Speaking to the hardware in the PS4 is like two native English speakers having a chat - quick, simple and efficient.
Speaking to the hardware in a PC is like a guy who only speaks English wanting to speak to a guy who only speaks German - the problem being he has to use an English to French translator and then a French to Spanish translator, and then a Spanish to German translator to communicate every sentence.
Of course - early PS4 games will probably use some familiar, common API layers (OpenGL etc) until dev's get time to get to grips with calling the hardware natively, so don't expect miracles from the first generation of 3rd party software!