Just returned from Xi'an. For a great guide & driver contact Mary ( email@example.com ).
She picked us (wife and myself) up from hotel... was both our driver and guide for the full day. Cost was very reasonable. She did charge an additional small fee for her guide service (this is optional, but well worth it). It was about what we would have paid if we hired a guide for just one site... and she acted as our guide for the whole day. We visited three or four sites each day. The only additional cost was the entrance fees at each of the sites we visited... but we did not have to pay for Mary's entrance fee, because as a registered guide, she has a pass that gets her access.
You can tell she is proud of her heritage and proud of her city (Xi'an).
Good guide/drivers that speak very good English are hard to find in Xi'an. She is very pleasant to spend the whole day with and very knowledgeable.