I'm a hardware noob, and I have tried to read a lot more than I have understood.
My current motherboard is a Socket 745/Athlon 64 with three memory slots. I read that if I actually used all three slots, the memory would run slower.
Now I'm about to upgrade to a P45 based board (probably a Gigabyte) with a E8400 CPU, and I'm wondering if there are any similar restrictions on modern boards. I.e., can I fill out the entire 16 GB that the board allows at the RAM's maximum speed?
Also, the board appears to support DDR2 RAM at 1066 MHz (and some versions even at 1200 MHz). I read someone saying that anything above 800 HMz is wasted due to the speed of the FSB. Is this correct?
You are correct in that it's linked to the FSB, and stock FSB speeds with Intel CPUs are limited to 333. If you are running at stock speeds you will only need to run your RAM at 667Mhz.
If you overclock a CPU, you might bump the FSB up to 400. If you overclock more, say 425 FSB, then you need memory faster than the 800Mhz.
When a board advertises for instance 1600Mhz, it's just talking about a 400Mhz FSB. It's a statement about how much you can overclock.
Most P45 boards will run well in excess of 400Mhz FSB. It's actually incredibly easy to overclock an E8400 to 3.6Ghz (400Mhz FSB) on a P45 board. Frankly, it as if the CPU were underclocked to start. Given that, and the price of RAM currently, it makes sense to buy 800Mhz RAM, at least.
Putting 16Gb on a board is tricky. If you want to do this I recommend you look up the QVL on the Gigabyte website and find memory that was tested to work in that amount and config. Generally, only single sided DIMMS work, and it can be hard to shop for those because the manufacturers don't list that in the specs.
Thank you for the answer. I feel quite enlightened
I'll probably not go for 16GB from the start, I just want to make sure that the option is there for later (so I won't get bitten like I did with my current board). I take it that I should take the same precautions if I fill all four slots with lower capacity modules as well, if I want them all to run at their optimal speed?
You correspond 1600 MHz RAM to a 400 MHz FSB (a four times multiplier), but say that the limit for the stock 333 MHz FSB is 667 MHz RAM. Is this because overclocking also increase the multiplier?
It seems there is plenty of high-speed RAM in the shops. Are they only used in overclocked machines? Or is it just Intel CPUs that can't use them directly?
ALL DDR2 has a multiplier of 2. So, it runs double the actual FSB. In the case of a 266 FSB, that means 533. 333x2 is 667Mhz RAM. You can increase the multiplier of your RAM (In some cases this is called CPU/RAM ratio) but you can never run it at less than x2.
It's all just various simple formulas. Some boards will actually only list the "quad pumped" figure, say 1066, and apply some other multiplier to arrive at RAM or CPU speed.
In some cases a BIOS will deal with RAM speed as if it's completely divorced from the CPU..
Anyway, just remember that you are dealing with a real FSB that is 266 or 333, or overclocked at some point beyond. Everything else is multipliers, even if they are not whole numbers. (A cpu might have a multiplier of 7.5, for instance)