As mentioned, this will depend upon your camera. For instance, the older cameras cannot take advantage of the new UDMA/300x cards. While they can USE them, they won't benefit from the fast speed because they are not set up electronically to do so. For instance, a couple of months ago I bought an 8 GB UDMA card in preparation for a trip. The card worked fine in my D200 and D2Xs but neither camera could take full advantage of the card's speed. Only when I put it into a D300 or D3 did I reap the benefits. HOWEVER, it's not just the camera in which that speed is relevant; it also matters in uploading images to the computer. If one has a UDMA card reader that will quickly upload the images into the computer, much faster than a regular card reader will.
Look in your camera's manual to find out the recommended cards for that camera. You can usually safely utilize faster cards but only up to a certain point.