The vast majority of Nikon digital cameras use the CompactFlash cards to store images. The CompactFlash card is about the size of a book of matches and fits into the slot of the camera.
There are two types of CompactFlash cards, Type I and Type II. The only difference between the two is that the Type II is almost twice as thick (3mm and 5mm respectively). The type of card you use depends on the size on the card slot in your camera. If you have the thinner Type I slot, then you can only use the Type I card. If you have the wider Type II slot, then you can use both Type I and Type II cards. Type II cards can be used in Coolpix models 5000, 5400, 5700, 8700 and 8800, plus the Nikon DSLRs.
CompactFlash cards range in size from 8 megabytes to 12 gigabytes. You can use any of the 8 megabyte to 1 gigabyte cards without any compatibility issues. 2 gigabytes and larger cards use the FAT-32 file system, and your camera must be able to read it or it will not work. Both the D70/D70s, the D2H/D2Hs/D2X and the D200 are compatible with large capacity cards.
Some manufactures give their cards ratings of 4X, 8X, 20X, etc. What does this mean? This refers to the speed of the data transfer rate, where 1X=150 kilobytes per second. The higher the speed rating, the faster the camera will be able to write images to the memory card.
CAPACITY (NUMBER OF IMAGES)
The number of images that can be stored on a card is based on the amount of memory on the card, the resolution of the camera and the image quality you select. A 3 megapixel camera with a 16 megabyte memory card shooting in High-Resolution JPEG will only hold about 13 images, but a 4 gigabyte memory card will hold around 3,000 images.
If you use a 5 or 6 megapixel camera and shoot in TIFF or uncompressed RAW (NEF), you will need a card big enough to store the images you shoot in one session. If you are not comfortable using a single 2 gigabyte card, try two 1 gigabyte cards.
If you have a D2x you should consider 2GB and larger cards, more so if you shoot RAW files.