Will DDR3-1333 4GBx2 work with preinstalled DDR3-1066 2GBx2 in iMac 27(i5)?

It needs PC3-8500.

Actually we don't know for sure..... maybe 1333Mhz will work... for example 800Mhz DDR2 So-Dimms work great at 800Mhz in white MacBooks way before they were officially supported in a more recent incarnation of the white MacBook...

The i5 & i7 Lynnfield chips onboard the latest 27 "iMacs definitely support 1333Mhz...

yohey_h

First welcome to the Apple Discussions.

The memory specifications for the 27" iMacs are here

As a rule of thumb _DO NOT_ vary from Apple's specifications. Your question has been asked many many times before for other iMacs, in short if you want your iMac to work reliably use what is specified.

Regards,

Roger

The i7 and i5 processors will handle both frequencies, 1066 and 1333 MHz, however there's no benefit if you mix the frequencies in one installation because the memory system frequency chosen by the processor will be the slower frequency found installed. In others words all memory would run at 1066 in the example you suggested.

http://download.intel.com/design/processor/datashts/322164.pdf

http://download.intel.com/design/processor/datashts/322165.pdf

yohey_h,

Good idea, however you don't have to purchase Apple memory. I would recommend either Crucial or OWC. As long as the specifications are the same it works just fine. Here is the link for OWC's solution:

http://eshop.macsales.com/shop/memory/iMac/DDR321.527

Regards,

roger

Here's a complete answer with all the details we want to know:
http://digg.com/d31BPxn

Neostar, thanks for the link to the article comparing performance of 1066 versus 1333 MHz RAM for the iMac (late 2009). Very little gain and likely not justified when cost considered. Kingston 1066 MHz 4GB memory sticks, for the suppliers I looked at, in Canada costs more than Apple Canada's memory; therefore the costlier 1333 MHz is even more difficult to justify. I understand the rest of the world seems to get memory so much cheaper so I only speak from the Canadian perspective.

Well I opted to order my Core i7 iMac (built to order) with an upgrade to 8GB RAM (DDR3-1066) for $112 here in Germany. Though I didn't order from Apple directly since they sell this upgrade for $270.

Swapping the whole RAM out for 8GB of DDR3-1333 (Kingston HyperX SO-DIMM Kit 4GB PC3-10667S CL7 (DDR3-1333)) would have cost me at least $302. Paying nearly 3 times the price to get to the same amount of RAM is something I can't justify.

At first I thought I made a mistake by not changing the whole RAM to DDR3-1333 but now that I've seen the results I'm pretty happy with my decision.