I use the w5100 and SD card together a lot. That is how I get my files into my SD card. I use a FTP program to transfer them to and from a FTP server. That program may be a bit too memory heavy to use on an Uno or Arduino Ethernet tho. That is why I have a Mega.
http://playground.arduino.cc/Code/FTP
That sketch uses two sockets on the w5100 (one command and one data) and the SD together. Smooth!! 