Using the D3100 myself: The Sony card I have seems to work just fine, as does the Samsung card that was given to me. My SanDisk card seems to upload to the computer faster, though, even though they're the same rating. No real stats behind that, so as we say in the biker world, that's a "seat of the pants" assessment.
I keep a rotation of 3 cards going at all times. One in the camera, a spare in the bag, and typically the last one I used still in the computer. When I import a new card, I rotate the spare to the camera as soon as I take the active card out of the camera, then the one in the computer goes to the to the bag. Helps keep me from finding that "OMG" shot only to suffer a facepalm when I realize I don't have a card with me (which I've done a couple of times). More than those 3, unless shooting large events, shouldn't really be necessary, but I have heard some talk of replacing SDHC cards once a year to prevent corruption and failures. If you find an amazing price, I could see picking up a couple of spares.