Using several PSU wattage calculators I found online, a 400 watt power supply should be enough for me (I want a little room to upgrade/add but not something too high). I am having a little trouble calculating exactly due to the fact that my Graphics Card is not one of the options on the calculators. I just wanted to check and make sure that was the case as it seems a little low to me. Here is my system:
-CPU: Intel Core i7-930
-Motherboard: ASUS P6TD Deluxe
-RAM: G Skill F3-12800CL7T-6GBPI
-Graphics Card: XFX HD-545X-YNH2
-Monitor: Asus VW266H
-(In process of choosing case)
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.