What is the difference between dual ranked & single ranked DIMMs

I'm pricing a server, and need to know the difference ...aside from the fact that single ranked is $600 more.
cerminadAsked:
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Gary CaseRetiredCommented:
Callandor -- the extra features are provided by the single-ranked DIMMs :-)

Single-ranked DIMMs have a slight access time advantage;  use a bit less power; and have better ECC capabilities.   But as I noted above, the "normal" reason I suggest them is in unbuffered memory situations, where the reduced loading on the memory bus is an advantage -- but this is not a factor with buffered memory.   Bottom line:  it IS "better" memory => but either will work fine.
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CallandorCommented:
The dual-ranked DIMMs have a few features which the single-ranked ones do not.  See the second page of this document: http://www.valueram.com/news/pdf/MKF_676_MarketNews_oct.pdf
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Gary CaseRetiredCommented:
See items #9 and #10 here:
http://www.corsairmicro.com/memory_basics/153707/index.html

Note that dual-ranked modules require an additional clock cycle to stabilize the chip select signal.

With registered (buffered) modules, either will be good, stable memory -- the only difference is in the possible feature sets, as noted in the article Callandor referenced.

Doesn't apply to your question (a server undoubtedly uses registered modules) -- but for anyone reading this in the future, with unbuffered modules the single banked modules have 1/2 of the loading on the memory controller (see #10 in my link) and will be more stable as a result.   That's why, by the way, many systems slow down the memory access speed (e.g. 400MHz memory runs at 333MHz) if you have 4 dual-bank modules installed.  (the degraded signal makes operation at high speed less reliable)
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cerminadAuthor Commented:
I also should have noted this at first... The servers being spec'd out are a Dell 1850 & 6850.  1850 being used as a Citrix server & 6850 will be a web server.  Should I go with the single-ranked DIMMs?
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CallandorCommented:
I would think they would be fine, unless you specifically need the features noted for dual ranked ones.
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Gary CaseRetiredCommented:
... to more specifically answer your question, "Should I go with the single-ranked DIMMs?" ==> I would;  but be aware that it's an incremental advantage and you're paying a fair number of dollars for it.
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CallandorCommented:
Gary, that's what I meant ... ;-)
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