Usually you can use any SD-Card. Some may work better than others.
What I recognized though, is the fact that "faster" cards can cut down page turning and book loading times immensely. I have a fast 2GB sandisk card in my camera which I tried out. The fasted times you get though is, if you put the books you read directly on the reader. I worked out a routine that works well for me. I put whatever I want to directly read on the reader. Once it's read and I want to keep it handy, I move it to the card. Collections I have created for the files still work if you just move the files.