The HD 5450 has been tested to consume a maximum of 13w of power, it should be fine. AMD's recommendation is based on the fact that they do not exactly know what is inside your PC. Watching videos will probably mean it uses 8w of power.
The Athlon 64 x2 4800+ is actually your most power consuming component. It uses a max of about 80w of power. For watching movies you don't have to worry about that though. It probably will use around 50w.