Hi, my friend and I will be visiting Chengdu from 20/11-28-11/2012. Heard so much about Jiuzhaigou.

Well, actually the best time for the traveling there should be around late Sep. to early Nov. As for your trip, it can be ok, but the scenery is that beautiful. If later, it may snow and the park may be closed.
During that time, it would be a little cold, with the average temperature of 13 C. Sometimes, it will be colder in the park. So I suggest you take along the overcoat and sweaters.