There are so many subpar guide experiences in China, but every now and then you encounter someone who really makes it worthwhile. For me, the major sights around Xian don't require a super-duper guide, but someone knowledgeable wouldn't hurt.

Last summer I had a friend who worked for the US Commercial Service: they MAY be able to arrange a tour with the VIP section of CITS. You can call them and try to request a guide (it will likely be expensive - perhaps 800 RMB per day for guide alone) but if you are able to afford it and enjoy the services of a guide, it would be worth it.

U.S. Commercial Service - Beijing. Tel: (86-10) 8529-6655