cm7 hasn't been updated much lately because of server issues then the concentration of effort on cm9. I've been using this kang for a few weeks and it's been working great -
http://forum.xda-developers.com/show....php?t=1344873
miui is very nice looking and stable but doesn't get updated much for the nc.
I've had my nc oc'd to 1300 for some time with no problems. Some people say there is no benefit to going above 1200. I've never tested the difference.
I currentlyl run a dual boot with base nook and the kang mentioned above. I also have miui on an sdcard. I love my nc as a cheap and dirty android tablet.