Xi'an private guides
 

Can anyone recommend a private guide for touring Xi'an, one that takes you in a private air conditioned car and speaks good English?

Virtually any travel agency will be able to arrange a private guide for you. Even if your agency is not Xi'an-based, IME the agencies have networks and can arrange whatever you need. The only issue is that guide quality can vary from fine to excellent.

If you are not working with a travel agency already, I can give you the contact information for the Beijing-based agency that I use. PM me if this would be helpful.

Agree...and likewise, the English proficiency of the guide will also vary, and the driver is unlikley to speak any English

Just curious... how much do guides in China cost per day?

My friend who recently had a one-day private tour in Xi'an paid 1200 RMB for a car, driver, English-speaking guide, airport transfers, and admission to the Terracotta Warriors, Big Goose Pagoda, and the famous hot springs (that I can't remember the name of). Lunch may have been included, too -- I don't recall. This amount included an understanding that she would not be taken to shops or restaurants where guides get kickbacks, i.e., that she would spend her time at the sites.

Not a complete answer to your query -- but for those who do visit the Terracotta warriors there are english speaking guides available. Your hotel can reserve them for you - we booked through the Sheraton. We had 'anna' escort us through the facility her english was terrific and she was well worth the 150RMB fee.

Unfortunately, we will be flying in just for the day from Beijing, so we won't have a hotel and would prefer a prearranged guide. I'm leaning towards using a travel agency in beijing, even though we are arranging the guides in beijing ourselves.

CITS VIP Section
 

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

Excuse my ignorance, but what is CITS? What is the VIP section?

Also is there any way to contact them via e-mail, otherwise I will have trouble coordinating it so I call during their business hours over there.

Having toured Xian myself this past summer, I honestly don't think a guide is necessary. Taxis are cheap and easy (as long as you have the destination written down in Chinese). As our resident guidebook author will tell you (and I agree with him on this), you'll probably get more accurate information out of a guidebook than you will with a guide. That said, I've had some nice guided tours in China, and you might not even mind being overcharged for the service. :p

On the Frommers and Fodors China forums, folks will recommend Xian guides and tell you how much they paid for them. That should get you pointed in the right direction. If you take taxis instead, it will cost you a small fraction of that amount.