This is my custom made lcd+sd card module. I used a DFRobot Keypad Shield and some cheap SD Card module from e-bay. Watch out, this is "raw" : it may or may not work for you, but I got it to work :)
I had to tweak Marlin a bit in order to get it to work, see intructions...
New : Use the repetier firmware. It's really easy to configure.
The hardware part is straightforward (i think). File, cut, make things fit in there, use 4 pieces of M3 threaded rod and some nuts, just screw into the plastic.
Wire everything (make a hole in the back for the wires :) ), write down the pins you used for this (LCD+Buttons+SD Card).
NEW: Software part
Use Repetier, and read the lcd configuration part, it's really easy. :) I'll upload my files when I get back home