I always upgrade firmware, whatever the fix I need or not, just to maintain the camera in it's latest upgrade ( I also do the same thing with computers )! As for memory cards, I have a bunch of 8 Megs ones. I was never tempted to go further as I very seldom need to go further then the two fitted in the camera, even at friend's marriage (not as the photographer). On travels I take six of those with me and often, two only are used (I tend to be over precautionary ). Of course, while I shoot more then with a film camera, I don't overdo it and seldom use the fpses the camera allows !
Same for the battery, I have to of them and change them regularly to slow wear, but can't remember a time when I had to change batteries in the middle of something
So I guess you'll be very happy with the D3x, as I am