As is known to the macbook users, all pre-unibody macbooks have two RAM slots for SO-DIMMs (DDR2-SDRAM, PC2-5300, 667 MHz, 200 pins). Many macbook users may be confused about the possibilities of RAM upgrading on macbooks and macbook pros. This article tries to share some information about macbook RMA upgrading. Apple default configs are paired. To find out which C2D macbook you have you can check the FSB clock or the GPU: If you have a 667 MHz FSB and a GMA 950 you have a Calistoga C2D MB. If you have an 800 MHz FSB and a GMA X3100 you have a Crestline C2D macbook. Do not confuse FSB speed with memory speed! 800 MHz FSB MBs still use 667 MHz RAM. Check your FSB speed in Sys Profiler. Unibody macbooks have two SO-DIMM slots for PC3-8500 DDR3 RAM, 204 pins, 1066 MHz.
White macbooks have two SO-DIMM slots for various types of SO-DIMMs. Although you can install 2x2GB RAM in the Calistoga macbook (Pro) for a total of 4GB, the chipset has an addressing limitation of 3GB. This is not something software, the OS or firmware could change – it is an inherent limitation of this Intel Calistoga chipset. If you install 4GB in an Calistoga macbook (Pro) the system will recognize the total amount of RAM, but you actually won’t be able to use the last GB. Unless 2x2GB is cheaper (or more easily available) than an unmatched 3GB, there is no reason to install more than 3 GB in a Calistoga MB.
A final word on matched DIMM pairs. It is true that memory bandwidth is increased by installing matched memory modules. In actual benchmarks this is something that is most noticed in GPU-dependent tasks (like 3D games) on systems with integrated graphics. The reason for this is that the GPU has to use RAM as video memory (rather than using dedicated VRAM over a dedicated bus) which means memory bandwidth can become a limitation. Therefore, if you are interested in getting the maximum possible frame rate, you would probably want to install memory in matched pairs.
However, this doesn’t mean that less matched memory is better than more unmatched memory. If your system is paging out to disk because not enough memory is available (this is something you can find out by looking at Activity Monitor’s “System Memory” tab) this will cause a huge performance decrease. This effect is orders of magnitude larger than the slight performance reduction due to unmatched memory pairs. Therefore: If you’re paging out with 2 GB of matched memory, DO NOT hesitate to upgrade to an unmatched 3 GB.
Of course going to 4 GB would be even better. Both the unibody MB and MBP are specified by Apple to support up to 4 GB of RAM. The MB and MBP however have already been shown to accept, recognize, and use 6 GB of RAM (one 2 GB and one 4 GB SO-DIMM). It will boot and run with 8 GB (two 4 GB SO-DIMMs) too, but as of right now, it is not stable. It has been rumored that a future firmware update could change this. And finally, it is your choice to upgrade it or not. But it is a pity not to create.