PC2-4200 CL4 vs PC2-5300 CL5?

I'm an IT Manager and we're about to upgrade a bunch of Dell D820 laptops from 2gb to 4gb(or 3.25gb due to the infamous 32-bit memory limitation problem) for the many vmware machines we run. Memory prices on large memory sitcks seems to have plummeted lately.

As of right now, we have the choice of getting PC2-4200, CL4 2gb dimms, or PC2-5300, CL5 2gb dimms. As it is, most of out laptops already have 1x 2gb PC2-4200 CL4 dimms in them. So my question is, does that increase in CAS latency lower performance to make the clock speed increase negligible? My limited understanding of CAS latency tells me that even though memory timings is a complicated subject, a lower CL is generally better. Also, for dual ranked DIMMs, would it be a problem mixing these clock speed ratings and brands? We want to be sure to get the dual rank performance boost but I wonder just how identical the two chips need to be to ensure this happens.

Thanks for your help.

BluespringAsked:
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Gary CaseRetiredCommented:
The discussion r.e. whether or not your system supports 667MHz memory is interesting (and the most accurate comment regarding that is jamietoner's last comment ==> the D820's can support both PC2-4200 and PC2-5300, but will only gain performance using PC2-5300 modules if the CPU is a 667MHz FSB model) ... HOWEVER ==> Nobody has addressed your fundamental question:  "... does that increase in CAS latency lower performance to make the clock speed increase negligible?"    

The answer is NO ... the higher speed memory is still the best for performance.   There are several reasons:

(1)  First (and most significant), the latency of the PC2-5300 modules is LOWER than the PC2-4200 modules !!   Note that 5 clocks at 667MHz is a delay of 7.496 ns, and that 4 clocks at 533MHz is a delay of 7.50469 ns ==> essentially the same (but the latter is slightly longer).

(2)  Independent of that, once a specific column has been addressed, multiple transfers from that row will happen at a faster speed with the 667MHz modules.

(3)  A lot of memory references are from the cache controller as it fills up the CPU's cache.  These all take advantage of #2 (multiple fills from a row) ... and, in addition, any reference by a running program that results in a cache "hit" occurs at cache speeds --> external memory speed makes NO difference for these.

Bottom line:  There's no technical reason to not buy the PC2-5300, CL5 modules.   Even if your system doesn't support 667MHz modules they'll work fine (at 533MHz).   And if they're running at the lower clock speed, they'll support a lower latency setting of 4 clocks ... so they'll work identically to the PC2-4200 modules you're comparing them to.

As an aside ... "matched pairs" is a nice way to package memory;  and it DOES guarantee identical SPD data .... but the real requirement for modules to work together in dual channel mode is simply that their SPD technical info matches ==> it doesn't matter if they're the same manufacturer, etc. ... only that they have the same technical specs.   It IS, nevertheless, a good idea to simply buy both modules from the same manufacturer ... and if they happen to offer "matched pairs" it's fine to use those.
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jamietonerCommented:
To get the performance enhancement of dual channel memory you'll want the memory to be in matched pairs(same speed, same rank, and same clock latency's). For what memory to get it depends on the FSB of the processor. The PC2 4200 will work better with a 533FSB and the PC2 5300 would work better with a 667 FSB.
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Glen A.IT Project ManagerCommented:
In the D820's it makes no sense to use PC2-5300 / 667Mhz...  unless they're less expensive.

Reasoning:  They will still run at the speed of the PC2-4200's already installed.  Your computer is currently configured to use 533MHz Front Side Bus, so there's no benefit to adding the PC2-5300 that runs at 667MHz...
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Glen A.IT Project ManagerCommented:
Also of note, if you want to use the performance boost, as jamietoner said you'll NEED matched pairs.  That doesn't just mean you use one PC2-533MHz module from one company/manufacturer and another PC2-533MHz from another....

Matched pairs means using the exact same part # and preferably from the SAME production run even...
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jamietonerCommented:
"In the D820's it makes no sense to use PC2-5300 / 667Mhz...  unless they're less expensive." Just because it has PC2-4200 in it now doesn't mean it doesn't have a 667FSB. Dell shipped laptops with a 667fsb and 533Mhz memory. If it has the core 2 duo with the 667FSB then you'll want the PC2-5300. The D820 supports core 2 duo's with the 533 and 667 FSB it also supports PC2-4200 and PC2-5300.
http://support.dell.com/support/edocs/systems/latd820/en/ug/specs.htm#wp1057468
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GoBieNCommented:
As a side note, if the PC2-5300 runs at lower speed you'll have to manually adjust the CAS latency to CL4, standard it would always stay at the value the SPD chips sets default, this case CL5.
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Gary CaseRetiredCommented:
Actually, SPD data is a table that shows the designed values for several speeds (all of the common speeds the memory supports).   If 4 clocks at the lower speeds provides sufficient delay, the SPD would almost certainly report a timing of CL4.

You can see the SPD data your module reports on the SPD tab of CPU-Z => it will show it in columns with the values reported for each speed.  http://www.cpuid.com/cpuz.php
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GoBieNCommented:
Oh, sorry didn't know that, i'm learning every day. I tought the SPD would suggest one value not matter what speed.
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Glen A.IT Project ManagerCommented:
Just to re-address the issue of whether it supports it...  We have over 400 D820's that are Core 2 Duo's, but they are ALL limited to the 533MHz bus...

So folks need to look at what they have...  in the case of these Dells, it totally depends on the D820's in question.  Why companies don't change model #'s when the computers are different I will NEVER know...  
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