If you have time for only one museum in Xi'an, this is it. The collection runs from prehistoric jade and oracle bones through Zhou bronzes, Han burial figures, Tang gold and silver, and a small annex of original terracotta warriors painted in their original colours. The display is well-organised and chronological, with English captioning that is much better than the Chinese-museum norm.
The free general-admission ticket gets you the main galleries; a paid ticket adds the special Tang treasure exhibition, which is worth the small extra. Go early — the queue for the free tickets builds quickly and is closed when the daily quota is reached. The museum sits a short walk north-west of the Big Wild Goose Pagoda, so the two combine well into a single afternoon.