What size microSD card do you currently have? You can re-format your current one if you haven't already after rooting your NC. Otherwise you can purchase any microSD card and just replace it with the one in the current microSD card slot. I just purchased a 16 GB Class 10 recently, super smooth. Heres the link from devs portal to reformat your microSD card NookColor Format and partition your SD card back to a usable state - nookDevs
Currently have a 4G and want to move to at least 8G, perhaps 16G. My concern is that I have backed up to SD and have some programs installed to SD. Thanks 4Nails