Memory: CPU & VGA

As I know, memory bandwidth, should be the same as the FSB.. Which makes sense, so that neither would be the bottleneck.

In example: CPU Core speed: 333Mhz, thus FSB(x4) 1333Mhz, would work best with dual channel DDR2 667Mhz OR Single DDR3 1333Mhz(latest option is a waste of course, but..).
Let's say, we're currently speaking about such a system: FSB 1333, Dual channel DDR2 667. & some 256mb VGA.

I ain't sure, if RAM is connected directly to the CPU.. Furthermore, I'm quite sure they're bridged by the northbridge. And, the northbridge has also the PCI-E friend connected to it, among with RAM & CPU. And, if the VGA runs out of memory, in certain cases it may use the RAM installed on motherboard(or maybe, it's still happens threw the CPU?). If this happens, and both CPU & VGA are requiring the data stored in the RAM.. And are both trying to access it, RAM might be the bottleneck: PCI-E, as I know has frequency of 2500Mhz(100*25?). So, the CPU either drains all the bandwidth, or it is split among CPU & VGA.. In any case, our 1:1 RAM, turns in to a serious bottleneck here.

So, could it be, that in such a case 800Mhz RAM would perform faster?