I believe the average size for a fine JPEG is about 2.2Mb so you'll be getting around 450 of them on the microdrive.
On my D1 the rear LCD also shows the number of shots remaining, and since this has four digits it still works when the top LCD is showing FL - meaning there are over 100 shots remaining. I'm not sure this is explicitly mentioned in the manual, but I'd expect your D1x to do the same.
One concern I would have with the microdrive, but don't know about for sure, is the battery drain. Being a microdrive, it has moving parts and therefore will probably eat up battery power more quickly then memory cards.
If this is true (JRP would probably know), then extra batteries might also be advised.