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.