I got a 8GB SDHC card, since my D40x creates very large images and can only hold about 500 in large (10MP) JPEG and less than 200 RAW images on my old 2GB card. The SDHC card worked perfectly in the camera and can hold about 600 raw images or about 2000 large JPEGs.
However, when I tried to read the card on my Mac, I found that my Griffin ExpressCard/34 5in1 reader won’t recognize it. I also tried an older USB multi format card reader, which also wouldn’t recognize it. Fortunately the USB reader that came with my EyeFi card is able to read it.