The right way to determine the proper size needed is to research all the components, determine their
maximum power demand, add them all up, and that is the
minimum supply you should get. But that takes a lot of time and work. So I recommend using a good PSU calculator and the best and only one I use or recommend is the
eXtreme OuterVision PSU Calculator. This will calculate your minimum needs and recommend a suitable size for those needs. It will also suggest an appropriate size UPS - something I recommend be used for all computers.
Plug in all your current components. Be sure to plan ahead and include all the hardware you think you might add in 2 or 3 years (extra drives, bigger or second video card, more RAM, etc.).
I recommend setting CPU utilization to 100% and Computer Utilization Time to 16 hours per day. These settings will help compensate for component aging, and add a little extra padding to the results. This will also result in a little cooler and quieter operation.
Note that no calculator wants to recommend a PSU that is underpowered so they all pad the results, some more than others. The eXtreme OuterVision calculator is and can be the most conservative for 2 main reasons. (1) They have a team of researchers on staff constantly researching components for us to keep their extensive databases accurate and current. And (2), it is the most flexible and has the most extensive databases of available options you can enter. This allows it to factor in all possible components to accurately calculate our needs rather than guess.
The eXtreme PSU Calculator ensures you buy a properly sized PSU for your needs. One that has a little extra headroom for future upgrades, but not one that is WAY too big and a waste of your money.
As seen
here, a nice 400W supply will work fine. Note those results are even a bit more overstated because I used the 5600 XT instead of the 5500XT. I also added a DVD drive, 3 x 140mm case fans, as well as bumped up the utilization times for even extra measure. So if me, I would get a quality 450W supply. If I might do some significant upgrades in the next couple years, then a 550W. Bigger than that is way more than you need.
I also recommend getting at least an 80 PLUS "Bronze" if the budget is tight. But if possible go for "Gold". Platinum and Titanium are not worth the extra cost unless you happen upon a "too good to pass up" sale.