There are a couple of cards available, but this one worked perfectly well for me; the R4 Revolution:
Right now the four gig HC micro SD card is NOT SUPPORTED. I bought it, and tried it, and couldn't return it. It may seem logical to get the largest possible card, but it will end up being a waste of cash. I hear they are working on updating the software, but I've also heard that it's categorically impossible to use the HC in the Revolution. That said... this two gig version will work PERFECTLY, and hold quite easily almost every game you could want to play, as well as precious Colors:
Finally following the directions that came with your R4, drag and drop the downloadable app with the extension .nds into a "ROMS" folder on your card... and blammo! Sketch to your hearts content, my little piglets!