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.

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.