Yes but not as powerful as high end PC. A lot of people went to upgrade their PC so that they can run Starcraft 2 in ultra even though the game itself runs fairly well on average PC but they want minimal lag, Fastest speed and maximum visual.
It doesn't need 4GB of RAM at all. http://www.computerandvideogames.com/article.php?id=189528 More RAM will allow other programs to run alongside it more smoothly though.
Cell processor is indeed powerful but the whole PS3 itself isn't more powerful than the a whole high end PC. RAM and GFX Card being the best example.
the ram is faster than PC ram as well, its just there isnt much of it. Which makes sense since XDR ram is incredibly expensive.
One of the reason why PS3 is not as expensive as a high end PC even though the Cell outperform PC CPU in for scientific calculation.
No. It needs a lot to run on everything set to ultra. Still, that doesn't even count a more demanding game like Napoleon:Total War or the upcoming Shogun 2.
It cannot use more than 2GB because windows won't allocate more than 2GB to a single program without a special boot switch.