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:

https://www.experts-exchange.com/questions/21342843/Dual-rank-VS-Single-rank.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!