Yes there is a difference. I use Extreme III cards and used to use Ultra II. There is a noticeable difference on cameras that are designed to take advantage of them. The Ultra II was 10 MBit/sec (appx 66x) and the Extreme III is 20 MBit/sec (appx 133x)
The price difference, of course, varies with the size of the card. But even for a 4GB, I think the difference is under $100 with the same reseller. (Probably about $75 to $80 with that one.) I find that the biggest difference between the two is in my card reader. I purchased the newest Sandisk USB reader and the Extreme III is significantly faster uploading to the computer. The repeated savings in time makes it worthwhile for me to have the Extreme III. With the D200, I'll probably purchase another (along with the 500GB external drive I just ordered!).
But it's still cheaper than film and processing in the long run.
I recently did a speed test using 1GB worth of D2X NEF images on Ultra II, Ultra III, Lexar 40x and Lexar 80x cards using my Lexar Firewire 400 card reader and copying to and from a SATA 7200 RPM drive.
The read speed of the Ultra III was about 18MB/s and write was about 11MB/s if I remember correctly (I have the numbers written down somewhere if anyone wants the exact figures). The Ultra II cards were about 10MBs-8MBs and the Lexar 80x were about 9MBs-7MBs.
So there was a clear speed difference with the Ultra III. However, unless you are shooting action sports and with the large buffer of the D200, is the price difference of the III cards really worth the money? For me, the only real advantage is transferring to the computer as it is almost twice as fast as the other cards. But this task is never really a huge rush and only serves to satisfy my impatient attitude when performing this action.