I just tried out the steps outlined here:
I used the 8gb image on my 16gb card, it just wastes a little space. There are a few other images for different size cards. I would use your FASTEST card you have (class 6 is recommended).
You just need to download the file, and then follow the steps on nookdev.com to put the image on your sd card (I used winimage on my windows 7 pc). It will completely erase your card.
Then, you just put the card in and go. There are few quirks w/ the system - it is still having some bugs worked out. But it seems quite usable. Great for playing around. Read the whole thread I referenced above for working around a few of the quirks (turn screen off then on to get rid of touch lag, turn keyboard off then on to get it to resize properly etc).
The nice think doing it this way, is it has very little impact on the actual nook (there is a temporary file that is created on the real nook in the cache where Nook stores temporary files).
If you like it, you can take that same card; download autonooter, and put that on the card (it will erase the 2.2 froyo image); then boot up with the autonooter, and it will root your Nook.
The test image is actually 2.2. and doesn't have ANY of the Nook software, so it won't be the same experience of rooting your Nook. Rooting your Nook is going to be more stable and have less quirks, but this is a great way to get your feet wet. I love my rooted Nook!