Well, when are you flying?
In the Summer (i.e., now) the jet stream is farther north and not quite as fast, and generally there are no convection storms out over the ocean so it is likely to be less turbulent then if you went in the late Fall or Winter.
You cannot expect a flight to be completely smooth (although I have been on them when they were) but the A330 is a great, large plane with modern amenities so it should be fairly comfortable.
Northwest also has a policy to fly around some turbulence for a smoother ride that other airlines would go right through. This of course could change with the rising cost of fuel...
I would recommend www.turbulenceforecast.com for more information on turbulence as well as links to the charts (easy to read ones) that are published by NOAA and other meteorological agencies that give you a generally idea where you might experience turbulence and how bad it might be. I find them reasonably accurate, but there are a lot of factors to consider.