What memory cards you buy can depend on what your main photographic interest is.
If you're into portrait or landscape photography fast, expensive cards are probably a waste of money.
If, like me, you're a sports/action/wildlife junkie, with a fondness for letting rip at 3.7 frames a second, then I would suggest at least 30mbs cards. There's not a lot in life that's more frustrating than having a red panda doing something cute right in front of you (something they're very good at, incidentally) and then have your camera lock up because the buffer is full.