Yes, it's correct that DDR2-800 (PC2-6400 / 400 MHz base freq.) memory is perfect for any CPU with x9 or x10 or x11 or higher multiplier.
It's perfect in the sense that it will not limit your CPU's highest practical overclock no matter what chipset motherboard you have since
many times the RAM has to run at least as fast (in the base input clock) as the CPU does (before the CPU multiplier and DDR multiplier).
Now Intel CPUs aren't 'designed' to be overclocked, so most CPUs have base clocks in the 266 MHz (FSB 1066) or 333 MHz (new FSB 1333 CPUs) area.
They do design the motherboards to use faster RAM than these LOW CPU default base clocks since it gives marginal benefit to be able to run
the RAM a bit faster than the CPU. That's the FSB:RAM multiplier which usually lets the RAM clock at various multiples of the CPU base clock.
A typical situation is to allow CPU:RAM options of : 266:266 (DDR2-533) ; 266:333 (DDR2-667) ; 266:400 (DDR2-800) and
occasionally 266:500 (DDR2-1066).
If the CPU/motherboard chipset supports 333 MHz 'default' CPUs for FSB1333 then typically you'd have to use at least DDR2-667 memory
which really has a base clock of 333 MHz since that's the slowest DRAM that has a base clock as fast as the CPU's base clock.
Multipliers available in that case would be CPURAM 333:333 ; 333:400 ; 333:500 (maybe) and so on.
But if you overclock the CPU then the chipset doesn't really change the available multiplier ratios for RAM, so the RAM speeds would proportionally
increase and you'd have to run the RAM at a slower and slower multiplier over the CPU base clock to stay within reasonable RAM speeds.
So to answer the question as to PC2-6400 (400) MHz memory being overkill for a CPU @ x10 (max), the CPU will be running at around
300 MHz (x10=3000) to 400 MHz (4GHz) range typically with a decently OCable CPU and good cooling.
So you'd skip 266 MHz RAM (DDR2-533 PC2-4200), and have say DDR2-333 (PC-5300)) running around 1:1 (CPU:RAM) if for example
the CPU can run 333x10=3330; or DDR2-400 PC2-6400 RAM running around normal speed if the motherboard has a 333:400 CPU:RAM multiplier.
You CAN lower the multiplier for any modern Intel CPU down to around x6, you just cannot raise it over their max. locked value.
People like high multipliers since it lets a low clock rate run the CPU to max OC without needing a motherboard or RAM that's supporting a
very fast clock rate to do it. Whereas if the CPU has a max. multi of x8, you can see you'd need quite fast RAM to even hit 3.5GHz on the CPU
since that'd need 500 MHz range frequencies for the RAM and Motherboard.
So in your situation you could end up with a maximum reasonable 380:380 range setting at CPURAM of 1:1 to get 3.8 GHz for x10 CPU multi
though of course you might be CPU/heat limited to less than that. That's where you get into the range where the
CPU clock is slowing down the RAM a bit because it's ALMOST the 400 which DDR2-400/PC2-6400 would run stock at, but too fast for the
next higher CPURAM multiplier to be used since that'd be something like (266:333) * (380/266) = 542 MHz actual RAM clock at the next
higher CPU:RAM multiplier you may have available for instance which would be too fast for most any affordable RAM.
So to answer your other question, YES, you can run RAM (or the CPU) at any reasonable LOWER speed than the maximum it's designed to support,
you just have limited room to go higher. If the RAM OC'ed by 10% over stock that'd be very reasonable. 20% RAM OC certainly possible but
not guaranteed.
Usually the RAM that's slower at stock speed OCs better whereas you get less OC out of RAM that's in the 400MHz or 500Mhz range to begin with.
So then you may consider dopping the CPU multi to x9 and try for something like 9x420=3780 with 400 MHz RAM; your CPU may not hit quite that
MHz, but even if it did it'd be a much more modest (i.e. usually achievable without instability) OC for 400MHz RAM, whle being more likely to
get the max. OC out of the CPU, with the advantage that the RAM wouldn't be slowed down much or at all relative to its own stock clock or
available OC by the CPU clock.
Thus you end up often running the CPURAM at 1:1 which has certain advantages for overall efficiency, and playing with the shared
CPU&RAM base clock in the 300:450 MHz range with various CPU only multipliers set to match the CPU and RAM speed.
Yes you could certainly end up with 400 MHz RAM being overkill for an x10 CPU that's never likely to hit 4GHz = 400:400 @ 1:1, but
you have to look at the alternatives.
Is 333 MHz (PC2-5300) RAM a lot cheaper than 400 MHz RAM? Not usually in today's market. There are exceptions, but there are some
exceptionally GOOD prices on 400 MHz RAM nowdays; it's probably the most commonly used speed for mid-range and enthusiast builds,
so it's also tending to be the best value.
Would 333 MHz RAM possibly limit the CPU OC? Well if the RAM could OC to 370, it'd probably just about max. out the CPU under
any reasonable expectation @ 370x10, so, no, it'd be fast enough.
If you buy 400 MHz RAM could you run it with slightly better cycle timings than 333 MHz RAM? Maybe, ok, so that's one slight possible advantage.
If you had the two options for basically identical prices for the same capacity, of course get the faster RAM.
Is 500 MHz RAM about the same price as 400 MHz RAM? No, the sweet spot of the market is 400 MHz or maybe 333 MHz now, and the
500 MHz stuff is like at least 33% more expensive and a lot rarer than the 400 MHz stuff, so get it only if you need it for an x8 locked CPU.
Save the money and buy either a faster CPU or MORE cheap 333 MHz or 400 MHz RAM.
The last factor is that the actual RAM SPEED doesn't matter much for common Intel CPUs. They certainly WRITE to the RAM a lot slower than
even DDR2-4200 RAM could support in dual channel just by the CPU design. They READ from the RAM faster, but, still, like 30% slower than
a comparable AMD CPU does with the same RAM. Intel's design decision is to offer CPUs with more CACHE memory, integrate better
RAM prefetch logic into the Core2 CPUs, and change the processor design so that it makes better use of slower RAM to get overall better
performance than the AMD CPUs tend to at the moment. So in that respect it's not a big deal whether you have 333, 400, 500 MHz RAM in
dual channel on a Core2 CPU with at least a reasonable amount of L2 cache. As long as you have ENOUGH RAM the performance will be
within a few percent to the application.
The main factor therefore is RAM PRICE, and AMOUNT of RAM, and making sure it's fast enough that you can OC the CPU to at least
around 3.5 GHz more or less since that's almost free almost guaranteed extra performance.
And avoid the junk RAM that's specified with slow CL6 or 5-5-5-23 timings or needs 2.1V to work and get the cheap
and plentiful 5-5-5-15 / 5-5-5-18 (best overall value) or 4-4-4-12 (usually much less of a good value) rated RAM that runs
at 1.8 or 1.9V and you're all set!
The bottom just melted out from under the 2GB ram prices, so it's hard to pass up $48 to $50 / 2GB stick prices to get
overall 4GB in dual channel identical DIMM model setups. That way it's plenty fast, plenty cheap for a mid-range build,
and more than enough memory to guarantee sweet application performance for years to come!
PS The motherboard chipset also lets you UNDERCLOCK freely and also LOWER multipliers freely in GENERAL. If you have
some DELL or HP or e-Machines or whatever really DUMBED DOWN BIOS maybe it'll give you ZERO control of clocks, timing, volts,
etc. and may even refuse to work with non-OEM branded memory/HDDs etc. etc.
But any even remotely modern chipset since the 965, 975, P35, nF4, 650, 680, with a reasonable BIOS that you'd get on any
typical modern DFI/ASUS/ABIT/GIGABYTE/etc. motherboard will give you at least some control of the timings, MHz, voltages, etc.
so you have no real motherboard based limits. I'd go with a cheap P35 chipset motherboard if you like Intel chipsets in this
day and age, personally.
Originally posted by: GundamF91
Great info QuixoticOne
So what you are saying is that DDR2-800/PC2-6400 should be enough for regular aircooled overclocked LGA775 processors? Same with E2180 which has factory 10x multiplier?
In case of CPU with 10x multiplier, what memory should be used, since 4Ghz is really impossible. What memory speed should the E2180 use? 10x400 would be 4Ghz. It sounds like either memory needs to be slowed down, or reduce the multiplier.
For memory speed to go down, I think factory default is 200mhz, to make default as 10x200=2.0Ghz. Would DDR2-800 be able to run at 200mhz speed? And doesnt' this mean DDR2-800 would be overkill for E2180?
For multiplier reduction, does Intel lock the multiplier from going down? For example, set the E2180 mulitiplier to 8x, but use the 400mhz DDR2, so it's 3.2Ghz, same overclock, but more bandwidth. In that case, would you use the DDR2-1066/PC2-8500 memory to up the FSB to 533Mhz and lower multiplier even more?
Also, is the above scenario limited by the motherboard's capability also? Thanks for any advice.