What you need to remember is that the manual for the D300 was written several years ago - somewhat before the camera was actually released. At that time the 300x 8GB cards were the best available. Nikon does not update the manual after it is initially released.
Lots of people use larger and faster cards in their D300 than are listed in the manual. The camera may not be able to take advantage of the faster write speeds that are available today, but the faster card can help reduce your upload time if you have a fast card reader connected to your computer.
I can highly recommend the SanDisk Extreme 400x UDMA 8GB or 16GB cards and the Lexar Professional 400x UDMA 8GB or 16GB cards. The 32GB and 64GB cards - and the 1000x cards - are probably just fine too but I have no personal experience with them.