A couple of weeks ago I noted an important new feature in the new MacBooks – the ability to use 6GB of RAM by installing one 4GB and one 2GB module.
Previous MacBooks and MacBook Pros were limited to 4GB of physical memory. Adding more was basically throwing away money, because the operating system wouldn't recognize anything more than 4GB. The cutoff for 4GB of RAM in a MacBook was the Santa Rosa upgrade, pre-Santa Rosa machines only support up to 3GB.
BareFeats dispels the myth that there's a penalty associated with running 6GB of RAM in the new MacBooks because they're an "unmatched pair:"
The only downside is that, with non-matching memory modules, you don't get interleaving. Potentially, there is a performance hit. But is the hit significant, negligible, or non-existent?
The bottom line? No problems. No penalty. BareFeats concludes that "you should have no worries about a speed penalty imposed by non-matching memory modules and loss of interleaving."
This directly contradicts what Apple has recommended in the past. In an archived tech note (TA38415), last updated in 2006, Apple warns MacBook Pro users to upgrade memory in matched pairs "for maximum performance."
if both slots are loaded with an equal amount of RAM... you can take advantage of the system's dual-channel memory architecture for an additional performance boost. With a dual-channel memory interface, both banks of SDRAM can be addressed at the same time.
Apple still recommends matched pairs of RAM in many of its MacBook DIY service guides.
In an interesting twist Other World Computing has confirmed that the 6GB configuration also works in MacBooks and MacBook Pros released November 2007 or later.
And in case you're wondering, 8GB of RAM doesn't work in the newest MacBooks (BareFeats tried it). Oh well. Hopefully we'll get access to a full 8GB (or more?) of memory in MacBooks running Snow Leopard. (Anyone tried this yet?)
Prices for a single DDR3-1066/PC3-8500 4GB SO-DIMM are still hovering in the US$600-$700 range, so you've got to really need the extra 2GB to justify the expense. Some prices from around the Web:
Surprisingly, Crucial doesn't carry the 4GB SO-DIMM in their inventory. If you've seen other prices, post them in the TalkBack.