Why DDR3 @ 1333MHz on Core I7 iMac + CAS Latency

Question

Hello,
I've just ordered an iMac 27" Core i7 (mid-2010) with 4Go.
On Intel's i7 page, I read :
http://ark.intel.com/Product.aspx?id=37148&processor=i7-940&spec-codes=SLBCK
Memory Specifications
Max Memory Size (dependent on memory type) 24 GB
Memory Types DDR3-800/1066
# of Memory Channels 3
Max Memory Bandwidth 25.6 GB/s
Physical Address Extensions 36-bit
ECC Memory Supported No
On the other hand, Apple says that memory upgrades shall match the followings:
* PC3-10600
* Unbuffered
* No parity
* 204 pins
* *1 333 MHz*
* SDRAM DDR3
My questions are:
1° Why shall we use 1333MHz since i7 only supports 1066MHz ?
2° As regards CAS Latency, if I add 2x2Gb with faster CAS, will they work at their own CAS Latency or at the stock 2X2Gb's ?
3° If I decide to leave one slot free, I can simply add 1x4Gb. Will this impact the dual channel performance of the stock 2X2Gb or will this perform as before ?
Thanks a lot for your help !
Cheers,

Answer 1

Hi yoms
Welcome to Apple Support Communities
{quote:}1° Why shall we use 1333MHz since i7 only supports 1066MHz ?{quote}
Do yourself a big favor and stick to [Apple's Memory specification,|http://support.apple.com/kb/HT4255] those that have not, have had all sorts of problems ranging from slow performance, Kernel Panic's, failure to boot, damaged slots and fried memory controllers.
{quote:}2° As regards CAS Latency, if I add 2x2Gb with faster CAS, will they work at their own CAS Latency or at the stock 2X2Gb's ?{quote}
When [Installing or replacing memory|http://support.apple.com/kb/HT3918] you can have 2 or 4GB modules in opposing banks, but they all must be the same latency and speed.
{quote:}3° If I decide to leave one slot free, I can simply add 1x4Gb. Will this impact the dual channel performance of the stock 2X2Gb or will this perform as before ?{quote}
More ram will help, but [Matched RAM on Intel Macs|http://guides.macrumors.com/MatchedRAM_on_IntelMacs] always provides the best performance.
Two highly recommended third party ram suppliers are [OWC|http://eshop.macsales.com/shop/memory/iMac/2010/DDR321.527] and [Crucial|http://www.crucial.com/store/listparts.aspx?model=iMac%203.6GHz%20Inte l%20Core%20i5%20%2827-inch%20-%20DDR3%29%20Mid%202010&pl=Apple&cat=RAM]
Dennis