The Eye-Fi card allows the camera to transmit images directly to any wifi enabled device - phone, ipad, or PC. It comes with a USB adapter to update firmware and set up the card. It has free iPad application for easy use on the iPad and iPhone.
I looked on Amazon for a Sandisk or alternative wifi SD card. Apparently it is a Class 4 card that came out of a joint venture between Sandisk and Eye-Fi. It may look cheaper, but there are some differences between the older class 4 card and the newer Class 6 and Class 10 cards. Both of the 8GB Eye-Fi cards - the Class 4 from Sandisk and Class 6 Eye-Fi Pro X2 were for sale on Amazon UK. The Sandisk product is not sold in the US.
One of the key features of Eye-Fi is that the Nikon cameras recognize it with a menu option to control the card. It is brand specific for Eye-Fi. Another key feature is the customer support for Eye-Fi is pretty good - nice to have when you are trying to figure out what setting you have missed that is causing a connection not to work. The newer class 6 and class 10 cards (the Pro x2) support NEF files.
You can probably get away with a 4GB secondary card, but the added cost for the Eye-Fi Pro X2 cards is relatively small.