efs555: first off... why did you post that in here? that has just about nothing to do with this thread at all, besides possibly helping. Second, i highly doubt it's DDR2, since that is heavily on the expensive side and still not very supported. Third, PCI Express still has it's bugs too. it's probably just the "not so perfect" hardware that's causing the slowdown.
Next time, please make your own NEW thread when you have a question. don't post your question in someone else's help thread, especially when it has nothing to do with the topic of the thread. i don't mean to sound rude, but that's kinda disrespectful.
As for my input about -seek-'s question, i agree with some of the others in here. Normally when you run sticks of different RAM with different speeds, it will run at the speed of the slowest stick of RAM. Using the same type, speed, and often make of RAM will be a much better choice.