Matched RAM on Intel Macs
From Mac Guides
The Intel Macs are capable of gaining some benefit from using matched RAM - where both memory sticks have identical specs (not just capacity, but identical in every respect).
The benefits are real but marginal. To quote from an article from barefeats.com:
"April 20th, 2006 -- Matched Memory Pairs make your Intel Mac Faster. Some of you were asking whether you would gain any speed using matched memory pairs on your Macbook Pro, Intel Mac or Intel mini. In our testing, iMovie renders were 3% faster with matched pairs. Our Photoshop CS2 MP actions ran 6% faster with matched pairs."
In cases where every bit of speed counts, there is a benefit to using matched RAM, particularly to those using professional applications, which tend to be RAM-intensive and where every second counts. The benefit may be slightly more noticeable on Macs with integrated video (MacBook, Mini), where the video processor uses main system memory (rather than dedicated VRAM). The speed of main memory response on those machines will affect video performance in all programs.
However, the benefits are marginal enough for most users that it is fair to say that the typical user will see no noticeable benefit from using matched RAM and will, in almost every case, see more benefit from buying larger RAM than from ensuring that the RAM is matched.
When will the extra RAM help?
If you are comparing a 2 GB (2 x 1 GB SODIMMs) setup with a 3 GB (1 GB SODIMM + 2 GB SODIMM) setup on a Core 2 Duo Mac, the 3 GB setup loses a minor amount of speed through the loss of Dual Channel. You can gain speed however, from having the extra 1 GB of RAM available to the OS and programs.
If you only ever use one or two small programs and keep the total RAM consumption below 2 GB, then you won't see much, if any benefit from the extra 1 GB. Where the 3 GB configuration gains speed is when you are multitasking heavily so you exceed 2 GB of RAM usage, and when you use programs such as Photoshop, VMWare, graphics, digital audio and digital video programs, all of which benefit from more than 2 GB RAM.
Early-model Core 2 Duo Macs can only recognize 3GB of the total RAM due to chipset limitations. Nevertheless, installing 2 x 2 GB gives a small speed increase through the use of Dual Channel memory access, as opposed to 1 GB + 2 GB. This may be worth it while the cost of 2 GB modules remains low.
Santa Rosa-based Macs (2007-2008) support up to 6 GB of RAM (although Apple officially says only 4 GB), while newer Macs can handle 8 GB or more.