I don't think you should trust anything thermaltake says, tbh. I don't.
Straight up real test showing the power used by a full system with a 460 installed in it. Measured at 227w at load.
Thermaltake says 537 watts because it is assuming you want to get the worst crap PSU at that wattage and some 550w PSUs barely give out more than 227 watts in reality. Like the Thermaltake kind, for instance.
Also look herehttp://www.jonnyguru.com/modules.php?name=NDReviews&op=Story3&reid=225
where the XFX 550w gets high marks in all categories by one of the two best testing websites out there.
Also here, earlier in the article where it shows the XFX 550w easily being able to do the whole 555w+http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story2&reid=225
The TX 750w can get to its labelled wattage too last I checked, and that is 200w more than the already 300w over XFX 550w. That makes it 3x of your actual usage.
Ideally, you want to target your actual usage to about the 50% mark on what the PSU is capable of supplying (for good PSUs like XFX what is on the label).
If you add another hard drive to the setup you showed and bump the wattage requirement up a tad bit more that lands you at about 500w targeted. XFX doesn't happen to have a 500w, the closest is the 550w.
They also have a 450w that would work, but it would give you less room to play around with and its better to fall under the 50% mark than over it.