It Is Possible - 6 GB of RAM in a MacBook or MacBook Pro
But is it sensible?
The official Apple spec. is that the new unibody MacBook and MacBook Pro models can address 4GB of RAM, however, we've been down that road before. For example, the Apple spec. for maximum supported RAM for the PowerBook 2000 Pismo remains 512 MB, but the Pismo will actually happily address twice that amount.
However, it's not quite so simple with the new unibody 'Books. Apparently, 4GB is not a hard ceiling, but you can't successfully double the official spec. either.
Tidbits' Adam C. Engst explains that to determine if your MacBook or MacBook Pro is new enough to address more than 4GB of RAM, run System Profiler and check the Model Identifier line in the Hardware Overview panel. If the first number is 3, 4, or 5, your 'Book should be able to handle 6 GB of RAM.
However, as I provisionally understand things, these machines get the optimum RAM performance with paired modules - eg: two 1 GB or two 2 GB modules, which take advantage of interleaving, the motherboard's dual-channel architecture that can increase the speed with which data can move from RAM to the CPU, so using one 2 GB and one 4 GB module to achieve 6 GB total eliminates the advantages of pairing. However, depending upon your sort of usage, you will still likely do better with 6 GB thanks to reduced virtual memory disk swapping.
Adam Engst says that that the MacBook and MacBook Pro both operate reliably with 6 GB of RAM, as long as it's the same type and speed of RAM, but unfortunately loading up with two 4 GB DIMMs for a total of 8 GB doesn't work well.
Weighing in on the same topic, BareFeats' rob-ART morgan reports that in hands-on testing he was able to expand his MacBook Pro 2.8 to 6GB by replacing one of the 2GB SO-DIMMs with a 4GB module from Trans International with no ill effects.
However, Rob says the 6GB configuration produced essentially identical benchmark results to the 4GB configuration on the 2.8GHz MacBook Pro - the difference raging from -.5% to +1.7% or an average difference of +.28% or less than 1/3 of one percent, which makes the substantial cost of buying a 4 GB memory module seem hardly worth it, even for fewer virtual memory swaps.
Rob also reports that the MacBook Pro will boot with an 8GB configuration (2 x 4GB), but there are serious operational issues.