In theory, a power supply with a specification of 5V 1A is more than ample for Raspbmc; however, this is often not the case. This is because the specifications printed on these supplies are unregulated, and are, in fact, printed and determined by the manufacturer. The best method of determining if a power supply is able to deliver 5 volts to the Raspberry Pi while under load (that is, playing a video) is by placing a multimeter between the points marked TP1-TP2 on the Raspberry Pi (shown in the following figure). By doing this, you can determine if the power supply you are using is delivering sufficient power.

A suitable reading under load is ~4.8V. However, this will vary on each device depending on the resistance of the polyfuse F3.