I've tried a few different cards on the D800, my fastest being a 60mb/s CF card, and just for fun I even tried the 16gb microSD card from my phone. No matter which card you choose, fast or slow, you'll be able to fire off at least 16 shots at max frame rate before the buffer slows down. It's only when you fill the buffer and continue to shoot that write speed becomes important. If you tend to shoot more than 16 shots per burst, or shoot multiple bursts with less than about 30 seconds between them, then get the fastest CF card you can.
For most of my shooting write speed isn't an issue as I rarely fill the buffer. I ended up getting some 32gb 45mb/s Sandisk SD cards, mostly because they were on sale for $34 per 32gb card. At that time a 32gb Extreme Pro was selling for $160+.
That being said, I've since found that write speed is an issue when using Live View as Live View doesn't utilize the buffer and the mirror doesn't return until the image is fully written to the card. The 60mb/s CF card takes about 3.25 seconds to cycle, the 45mb/s SD card takes 3.75 seconds, and a really old mini-sd card via adapter took 15 seconds. I don't have anything faster than 60mb/s. Perhaps someone with one of the fast (90mb/s +) CF cards would be so kind as to measure the Live View cycle time and post it here for comparison.
