Mixing DDR2 RAM - Asus mobo, Core 2 Quad CPU

No doubt this question has been asked 100 times before, but I am struggling to find a clear-cut answer that fits my case. Besides, it makes a change to be asking, rather than answering, a question in EE.

Here is the background:
I want to build a reasonable spec 64 bit server so I can test Microsoft Windows Server 2008, Exchange, Virtual server, MS-SQL 2008 etc. I have an Asus P5Q Pro mobo and a pair of 1GB memory sticks like this:
Kingston 2GB Kit (2x1GB) DDR2 800MHz/pc2-6400 Hyperx Memory Non-ECC Cl4 2.0V

For the apps I want to install the server will need at least 4GB of memory, possibly more. So rather than just adding another pair of DDR2's like the above, I thought I'd get a pair of 2GB sticks which would then give me the option of mixing both types of memory together to make a total of 6GB of RAM.

The plan is to use an Intel Core 2 Quad Q8300 2.5GHz (FSB 1333MHz) processor which would be purchased along with the new RAM (i.e. the choice is still open). I do not want to overclock anything - reliability is my top priority.

So my questions are:
1. How important is it to match each of the spec items: 800MHz, CL4, 2.0V and the Kingston manufacturer?
2. Is it safe to mix different speed, latency, voltage RAM provided the mobo is just configured for the slowest/lowest?
3. Is there any advantage to matching CPU FSB clock speeds and memory frequency to be in particular multiples (as I have read somewhere)? If so, would it be better to choose DDR2-1066MHz and not using the existing 800MHz sticks?

The memory I was thinking of getting is this (it seems to match the existing in all respects except manufacturer):
OCZ 4GB (2x2GB) DDR2 800MHz/PC2-6400 Titanium Memory Kit CL4(4-4-4-15)

Thanks in advance.
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feptiasAsked:
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CallandorCommented:
1 & 2) You can mix 800MHz RAM and different CAS latencies, but I would not advise mixing different voltage specs - the voltage regulator on the motherboard might not like that.  The 800MHz rating is standard, so there's no problem using different RAM with the same speed rating.  Different latencies mean the RAM won't be running as fast as it could, since it can only respond at the speed of the slowest.  You can manually set it or let the system try to, but it may get confused and try to run everything at the faster memory timing.

3) If you're not overclocking, there is no point in using RAM faster than DDR2-800 for cpus that use a Front Side Bus.  This is because a Core2 cpu with a 1600 FSB needs a 400 MHz motherboard bus, which will drive DDR2-800 RAM at its maximum rating (ie, the cpu is quad-pumped to the motherboard bus, while DDR2 is twice the motherboard bus).  The Nehalem i7 cpus do benefit from faster RAM, since they use a QPI and are not limited by a motherboard FSB.  If you're running a cpu with a 1333FSB, using DDR2-800 does not hurt you - the RAM will run at a slower speed.
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feptiasAuthor Commented:
Thanks Callandor.
1& 2) You've confirmed what I suspected regarding speed/latency, but your comment about the RAM voltage really surprises me. I have seen more than one reputable source - including Wikipedia - state that there is a standard voltage of 1.8V for DDR2. I assumed that ratings above this relate to the RAM's ability to work at a higher voltage and provide better performance (overclocking of sorts). The BIOS on my mobo also states that 1.8V is the default setting.

From a practical perspective of matching new RAM to old, the details available on the seller's web site don't specify a voltage for that OCZ package. However, the blurb states that "OCZ EVP (Extended Voltage Protection) is a feature that allows performance enthusiasts to use a VDIMM of 2.25V without invalidating their OCZ Lifetime Warranty". So I'm left a bit uncertain now if the OCZ will be ok.

3) So the 800MHz RAM would not be operating at its optimum clock speed with a CPU that uses 1333MHz bus. Is that right?
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CallandorCommented:
1.8v is the standard for DDR2; running it above this is an overclocker trick used to get RAM to work beyond its rated speed, but overclocking anything is pushing it beyond its expected performance envelope.  It might work, but it's not guaranteed to work.

>3) So the 800MHz RAM would not be operating at its optimum clock speed with a CPU that uses 1333MHz bus. Is that right?

The key word is not optimum, but maximum.  It works just as well at either speed; you don't suffer a penalty for it.  If you could spend less for DDR2-667, then it would be a better deal and functionally no different.
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feptiasAuthor Commented:
If I understood correctly, RAM rated at higher frequencies/voltages makes sense if you intend tweaking settings to get more performance out of it. Since that was not in my plans I decided to go for the safe option and buy some bog-standard Kingston RAM that is listed in the Asus QVL. It was about 10% cheaper than the OCZ too.

When it arrived I was surprised how thin and weedy these new 2GB DIMMs looked compared with the 1GB sticks I already have (from the same manufacturer). There's no heatsink on the new ones and they are about 1/4 or 1/3 of the height. However, they seem to work fine. I have only tried them on their own so far - not mixed the 2x2GB with 2x1GB to get 6GB. I may just abandon that idea now, buy the same again and double up to get 8GB because the applications I want to test use a lot of memory (Exchange server and Hyper-V).

Thanks for the help anyway.
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