Each "X" is 150 kBps
The big questions is what is the fastest the D70 can write ?
How big is the on-board buffer ? 4 RAW /12 jpg is what I've heard.
Here's a great resource for you:
http://www.photo.net/From a Photo.net messageboard:
"The highest write speed that you can get for D70 is around 4.5 MB/sec,..."
(on my calculator that's 30X - be sure that is a continuous-write rating)
The card speeds rated usually are Read, then Write-Burst speed, lastly continuous Write, as it recording.
24/96 Stereo is 1.1 Mbps, if you care. (about 8X continuous)
So a 40X should more than cover it, in theory.
See the Retail Space thread for a few bargains and reviews