Larry, I can't answer for the largest, I have used 32Gb with no trouble, but the fastest cards will gain you nothing much in the writing (ie clearing the buffer) stakes since the limiting factor is how fast the camera can write. This sets the limit at roughly 30Mb/s. The only advantage of faster cards is the speed at which they can be read using a card reader to transfer your images to computer. Although a potential second advantage would be, if you upgrade to a newer body which is capable of clearing the buffer quicker, the faster card might come into its own. In general, I tend to stick to max 8Gb cards which allow approx 400 images (RAW 12 bit lossless compressed) each. IMO this minimises the risk should a card go "bad" (as has happened to me twice, over the years)
Having just purchased a D800 I needed faster cards. I picked up the 32gb Sandisk Extreme CF and SD cards. I tried them with both cameras and they work fine. The D300s however does not take full advantage of 60mb/s of the CF cards. Fastest supported speeds based on research I did on Nikonians suggests speeds of 30 mb/s are fully supported and my testing agrees with that assessment. The D300s is a great camera btw. Still love it after several years and newer/better camera's!