I use a pair of 16gb Sandisk Extreme cards. The Class 10 is the current standard. Class 6 will work fine. 8GB would probably be adequate for photography - and will hold around 300 images. The camera will tell you it holds less - but due to compression of RAW images, you will get 300 or so under typical conditions.
If you shoot video or a lot of wildlife, a larger card is more important. I shoot 6 GB or less of landscapes in a day, but for wildlife will go beyond that.
Card speed is most important with downloads. The write speed in your camera is pretty fast, and the card will not typically be a limiting factor.
Be sure you buy from a reputable source. Avoid eBay unless you have a known reliable vendor. There are a lot of counterfeit cards from unreliable sources. Stick with known brands - Sandisk, Lexar, and Transcend. If the price looks too good to be true - it is.