Buying a prepaid SIM-card in Xi´An

Yes, you can buy it when you are in Xian. I am think which mode your cellphone supports, CDMA, WCDMA or GSM? If you want to use the internet service, you may ask the seller to give one that includes data packages. However, I don't think it's a good idea to use the one you bought in Xian when you are in HK. If you want to use it in HK, then you will pay very high price because of the international romaing fee. You actually can buy another one in HK because it's cheaper.