Microdrive's read and write speed is about 33x when it starts up, but when it's fully accelerated, it can get over 100x; HOWEVER, since pic files r 9MB max from a D70, it'll nvr speed up to full when saving pics from the camera. i used to haf a 1G Hitachi Microdrive, and now i switched to Sandisk Ultra II 80x cards. when i used the Microdrive, i can burst about 6pics w/ my D100, and w/ the Ultra II card, i can get appox. 2 more frame b4 it ran outa buffer. So i guess there's not gona b a noticeble difference between a 40x card and a avrg. 33x microdrive. i would go for a 80x card or even the Extreme III card if u really needs brusting, otherwise i would stick w/ microdrive.