fsb:dram ratio???what is merit about wht's the best ratio

[linkshivank]

hi guys!need a bit help here.
i have a pentium D processor and i'm looking for a new mobo+ram for it.
its a pentium d 930(800mhz FSB) and i'm caught in a confusion.
that how will buying a ddr2-800mhz ram affect my system than a ddr-400mhz ram
AS with the ddr-400 my fsb:dram ratio will be (800/4) : (400/2)=1:1
OR
and buying a ddr2-800 my fsb:dram ratio will be (800/4) : (800/4)=1:2

please help as i for any reason can't change my processor,so advise a board+ram combo by consider my confusion stated above.

[jlreich]

Forget about ratios, DDR2-800 is twice as fast as 400. ;)

[Heartborne]

If you're going to buy a new motherboard, why not just ditch that old processor altogether? A pentium d 930 makes a lovely antique these days, I'm afraid.
Of course if you're not looking to do that, you can run that in a motherboard that supports newer chips. What socket type is it? LGA 775 is the current standard and many motherboards that support the current generation of dual core and quad core chips (up to eight times faster than what you've got there) will support pentium chips as well. DDR2-800 is the current standard.

Some possible issues here are, of course, frontside bus speed related. I haven't done much playing around with this sort of thing, so I can't say for sure what your limitations would be as far as trying to upgrade your mainboard and RAM with a processor that is three generations old. FSB speeds are now going as high as 1600, and 1066 is standard.

Any of our more experienced members care to elaborate on how fsb speed relates to RAM speed? How the fsb of the mobo relates to the CPU? I'm curious about this myself. At what point does a ratio like Linkshivank stated begin to cause issues?

[jlreich]

First, the PD has a FSB of 800MHz, so you are perfectly synced at this point with DDR2-800. Although syncing doesn't really have to do with the end number but rather with ratios of the true clock speed of the ram and FSB, which in this case is 200MHz quad pumped to end up at 800MHz. Which happens to be in the case of the PD exactly the same. But also consider that DDR/DDR2-400 is also synced nicely because it is a ratio of 200 and that is what's really important. Add to it that DDR2-800 when used in dual channel actually becomes DDR2-1600 and it is still synced.

All that being said syncing really isn't that important these days. Modern memory controllers are very efficient and syncing is for the hard core tweaker looking to get every last ounce, or milligram is more like it, out of there system. The most important thing is making sure the ram has enough bandwidth to keep the CPU from starving for data. Which is really easy these days because of dual channel ram. Technically, for a CPU with a FSB of 1066 DDR2-533 is enough because dual channel makes it 1066. Likewise DDR/DDR2-400 is plenty for a CPU with a FSB of 800.

Why then do we recommend 800 all the time? One, because dual channel is not 100% efficient. Somewhere between 80-90%, so DDR400 is not going to add up to 800 in real life. Two, it usually doesn't hurts to have more bandwidth. And last but not at all the least it is dirt freaking cheap. :p

Considering this last statement when it comes to laptops DDR667 is usually the dirt cheap stuff, so I am usually quite happy to go with 667 in laptops.