Is it just me or is it pretty mindblowing what Moore's Law is doing to our computers these days? I remember when 64mb of RAM was great, and even a few years ago, I wondered why anyone would ever use 1GB of RAM (I've got 2GB in my gaming PC, and I still almost think it's too much). But apparently TidBITS has been doing some testing, and they've discovered that not only can the new MacBooks hold 4GB of RAM as Apple recommends for a limit, but stuffing a whopping 6GB in there is possible and doable. There are two DDR3 slots in the new MacBooks and MacBook Pros, and they come with either 1GB or 2GB in each DIMM slot.
TidBITS says you can switch out one of those 2GB DIMMs for a 4GB, and voilà, as long as they're the same speed and type, apparently almost everything is hunky-dory. TidBITS says there are two drawbacks: one, that dual-channel architecture requires identical DIMMs, so that's out if you're running a 4GB and 2GB configuration. Plus, it'll cost you an arm and a leg, especially if your leg is priced at around $600, which is what a 4GB stick runs. Even the DDR2 is pricey at that level (then again, if you don't like memory prices, just wait about five minutes, because that's basically how often they change).
And for some reason, there's an issue with running two 4GB sticks in there (for a total of 8GB), but TidBITS surmises that may be fixed by the time Snow Leopard rolls around. Personally, I'd love to see a program that needs 6GB of RAM -- maybe a high end 3D modeler or a financial simulator of some kind. But it's good to know that if you really need that much memory (and have the cash to spend on it), there you go.