I run my boot from the card so when I remove the sd card I just get stock nook color back. I used the instructions from the XDA forums for CM7. I have a couple of cards that I can boot from. I first used a 32GB but it kept freezing up and I'd have to reboot often. I switched to a 16GB - I think it is a class 6? - and I get much better performance from that one.
If you don't want to try the steps yourself, you can buy the boot cards from EBAY for a pretty reasonable price.