The 7210 is dual band GSM (1900 for North America and 900 for the rest of the world). It should work in Hong Kong - if your account is set up for International roaming and if Rogers has the proper roaming agreements in place (I'm sure they do).
Check the Rogers web site for international roaming agreement AND costs.
Voice will be expensive. Data should be OK - limit the browsing and instant messaging and you should be OK.