you should be able to. Think of the DDR2100 2700 3200 ect as gasoline ratings. A car that only require 87 octane fuel can run on 93 octane gas, but a sports car that require premium gas can't always run on regular fuel. DDR2700 (333) is just a more capable form of DDR. It CAN run at 333, but doesn't mean it has to run at DDR333, it can run at the slower speed.
So the answer should be "yes", you can use DDR333. But you just won't be able to take advantage of the higher speed ceilings of the ram. But that's why you overclock the FSB. A DDR266 might not be able to run at FSB much faster than the default 133 MHz, so your overclocking ability will be restricted. But a DDR333 will not be a OC restriction until you reach FSB of 166 MHz and beyond (and you will be very luck to be able to OC to FSB of 166).
I'm thinking about getting a GranitBay mobo as well, and I'm definitly getting a DDR333, if not DDR400 (so if I upgrade my mobo later, I might still be able to use the ram). But that's when the prices gets to be the problem. GB mobo is already expensive, and all that good ram isn't helping either.... money, you can never have enough.