Well your setup could run on a quality 300 to 350 watt PSU the HD 5450 barely draws 20 watts when its at full load. So a 400 watt PSU will be more then plenty for your RIG.
Now depending on if you want to upgrade in the future to a more powerful card 550 watt PSU would be a good choice to leave your options wide open for upgrades. A 550 watt PSU will run just about any single GPU card on the market except the HD5970 and the GTX 480 besides those 2 cards a 550 watt PSU is all you will really ever need.
Check out the link in my sig for Power Supply guide for graphics cards.