This is what I guess would happen:
If two cores tried to access the same address in RAM, one would have to wait for the other to access the RAM. The second time that each core would try to access the same address, they may still have that RAM cached, so they could access their respective caches simultaneously.
If two cores tried to access different addresses in the same RAM, one would have to wait for the other to access the RAM.
In other words, I would imagine that for RAM intensive programming tasks, multiprocessing won't help much unless it involved reading from the same address in RAM multiple times per core.
So, can multiple CPU's / cores access the same RAM simutaneously, or is what I'm saying correct?