You'd better keep in mind that if you go with the "cheap" cards that you are most likely limited to the 1800mhz band and likely to experience dropped calls. I know a lot of people experiencing dropped calls with a lot of cheap SIM cards sold in Mongkok's streets (the street vendors).

I settled for a SmarTone card because I didn't want dropped calls since they operate in both 900 and 1800mhz. Never experienced a dropped call. Most 7-11 and other grocers and cell phone stores will sell them.