The PS3 doesn't have the fastest RAM speed. But it still has the fastest bus between CPU and GPU on the home market. The fastest PC (Pci-e 2) isn't even close. The future Pci-express 3 (32 GB/sec) will come close but still won't reach the speed of PS3. The problem for PS3 is it can't hold all info in video RAM as well as main RAM needed to get the same graphical quality as the PC. In other words it can't store all readily available info needed to render a single frame. So in a situation where the PS3 needs more info to render a scene as intricate as what you'll find on the most powerful PC the memory runs out and it has to resort to read from the much slower HD.Frak... RAM is cheap and PS3 isn't setting any memory bandwidth records. See the first part of my previous message for the rest.
On the physics side the PS3's CPU Cell crushes any CPU on the PC side. You need a GPU on the PC side to compete with cell in this regard (and a new GPU is considerably more powerful than Cell here).