Some time ago I bought a little 13″ white MacBook of the “Mid 2009″ MC240LL/A (MacBook5,2) variety that, according to the specifications, was already at its maximum supported memory configuration of 4Gb. However, after reading about a firmware update that somehow magically enabled greater memory capacity, I decided to try it for myself.
I purchased 2 x 4Gb DDR2-800 (PC2-6400) SO-DIMM Kit (Corsair Value Select VS8GSDSKIT800D2) and installed them in place of the previous 2 x 2Gb units. The machine booted first time perfectly happily with 8Gb RAM in use. Success!
I’d also read that some machines would boot with this much memory but suffer a performance decrease, so I took the precaution of taking benchmarks before and after the upgrade. Fortunately, these benchmarks actually showed a decent performance gain of around 8% overall.
The key, it seems, is to have the correct version of the Boot ROM and SMC firmware.
This little hack worked well for me, but as ever with these things, your mileage may vary. I don’t know if there are specific (unlabeled) revisions of the hardware that will or won’t support this much memory, or support it but suffer performance degradation or other problems. Caveat Reader.
However, I can confirm it is actually possible for at least one MacBook.