Welcome to Nikonians! I have used 4 and 8GB Extreme III 45MB/sec. cards in my D200 for years without issues. I have a few 32GB Extreme 60MB/sec. that I have used in my D200. The 4GB Ultra II's are selling for about $12.00 and the 4GB Exteme 60GB/sec. cards are selling for $24.95. You can buy a 2 pack of 8GB Extreme 60GB/sec. cards for $59.00.
Since you plan on upgrading your camera, if you are thinking of a body that uses CF cards, get the the 8GB Extreme 60MB/sec. cards. If your next body uses SD cards, get the 4GB Ultra II's. Good Luck and Enjoy your Nikons!