D90 can't put images to the card fast enough to take full advantage of the fastest card speeds. However, I have no idea about video recording, a faster card might be valuable there (though I seriously doubt it). Now if you had a D3 or a D700 and grip and wanted to shoot continuous bursts of RAW files on the high speed setting, you may want to invest in a faster card; but even then you'd need to be mindful of the camera's max write speed.
SLC