Question : Problem: Dual vs. single ranked memory RAM - which do I want?

I am getting a Dell PowerEdge 840 server and they offer single and dual ranked ram and this is sometihing new to me.

I saw this question:

http://www.experts-exchange.com/Hardware/Q_21342843.html?query=dual+ranked+&topics=45

but that doesn't seem to answer - which one do I want?

back in the day, I think the thinking was - better to get more RAM on a single stick, to leave room for more sticks later?

(if you needed 512 MB, you might likely opt for 1 stick of 512 rather than 2 of 256... because older machines only had 2 or 3 slots?

But then there was DDR2, which, other than a single stick, you had to put ram in there in pairs?  so would 2 -256s be better than 1 - 512 then?  there would be performance issues, but you were still eating up slots faster 1 way rather than the other....

But now for the server:

you can get:

  1GB DDR2,533MHz,2x512MB Single Ranked DIMMs add $0  
 
   1GB DDR2,533MHz, 1x1GB Dual Ranked DIMMs [Included in Price]  
 
Which do I want?  for Performance reasons?  For expandability reasons?   WOuld the answer differ if this was for sbs 2K3 standard or win xp pro?

THANKS!

Answer : Problem: Dual vs. single ranked memory RAM - which do I want?

A "rank" is a 64-bit wide group of memory modules that contain a particular amount of memory.   It does NOT have to correlate with a "side" of a module ... but with unbuffered modules it almost always does.   With registered modules there are single-sided dual-rank modules; and double-sided quad-rank modules; in addition to single-sided single-rank and double-sided dual-rank modules.   But for virtually all unbuffered modules I've seen the # of sides equals the # of ranks.

In general, it's best to use all single-rank modules ==> the bus loading is 1/2 of that for dual-rank modules, so the memory subsystem (for unbuffered modules) is more reliable.   But high-capacity single-rank modules are very expensive ... so that isn't always an option.   ... and it really doesn't matter if you only have two modules installed.   The high density modules that allow you to add, for example, 4GB of memory using only 4 ranks (2 dual-rank 2GB modules) are very expensive => note that for the PE 840 it would cost $2619 to get 2 2GB modules; vs. $509 to get 4 1GB modules (but you'd then be using 8 ranks ... with double the bus loading of the higher density modules).

Enough details ... if you don't plan on moving beyond 2GB I would buy the single-ranked 512mb modules ... you'll get dual channel memory operation, and will only have a total of 4 ranks when you eventually add two more single-ranked modules; or at least no more than 6 if you added higher capacity dual rank modules.