Definitely get a private guide. In 2008, we hit eight cities in 20 days with 10 internal Chinese flights with a new guide and driver in each locale.

Xian has lots to see. The Terra Cotto warriors, although large, are dwarfed in number by another tomb (don't have name handy) hear the airport. In addition, there are a couple of pagodas to see/climb, an interesting mosque, and a wonderful market area.

I think Chengdu is wonderful. In addition to seeing the pandas, are night at Sichuan opera is probably best cultural show we have ever seen. We also saw Le Shan, the largest carved Budddha in the world (now that Bamiyan is destroyed).

Another option is Tibet; well worth the effort.

China is a wonderful country to visit. We still have places we want to see and would love for the opportunity to go back.