In addition to the fancy western chains you should look at Jin Jiang Inns -- it's a chain of very nice hotels with more than one location in Xi'an.We stayed there, it was modern, clean, good service, nice breakfast, fast internet, great experience. See http://www.travelchinaguide.com/hote...efang-road.htm
Taxis are famously annoying in Xi'an. Take the subway or the bus instead. See http://www.chinatouronline.com/china-travel/xian.html for some pointers.

Oh and don't just go to the terra cotta warriors. Few people realize that there's an even more spectacular buried-soldier site nearby -- the Hanyanling mausoleum. The soldiers there are smaller, but more beautifully done and well preserved. The above website has good descriptions of all the many things to see in Xi'an. We actually found the Daming Palace site to be one of the highlights of our visit.