You link failed to show the test system specs, Tweaktown's fault, so I don't know what components other than graphics card made that number.  Remember, semantics matter.
 
FYI, most people here will recommend more on the basis that 1) you intend to upgrade more components anyway and 2) running a PSU at peak power is, while generally wears the unit down much more, tend to have a peak efficiency between 50-80%.  Furthermore, people will treat "system requirements" as a minimum and the whole system will fail without it, when neither nVidia nor AMD actually tell us what configuration they quote power from.  
 
In other words, recommendations are not remotely scientifically related to what the graphics card is actually using.
 
For instance, let me link a guru3D review of an R9-290X, a more powerful card than yours.  Their test system includes a 32nm SB-E 6-core overclocked to 4.6GHz, which is at least triple your Ivy Bridge quad-core.  They claim the 290X uses 284W for itself while they recommend a PSU in the 550-600W range.  You, of course, have a regular R9-290, so you may not need to downclock it with the PSU you have in mind.  That said, should you want a better PSU, you don't need to overkill unless you plan on more components later on.  A 650W unit would be great, though nVidia quotes 600W as a minimum for all of the 250W cards from GTX780, Titan, GTX780 Ti and Titan Black.
 
But if you want to cut it close -- I don't recommend this-- keep in mind most of the power drawn by a computer is from the 12v line, so you can't just take guru3d's 300W-ish value for system full load with the 32nm 6-core and R9-290X and think a 300W PSU would do the trick.  That is 300W-ish from 12v, which comes to about 25A-- you need to find a PSU with at least that many amps from the 12v line and it will be above 350-400W no matter what.
 
So if we treat the Tweaktown number as accurate, that wattage is from 12v only, 450W from 12v is 37.5 Amps-- does your PSU meet the amperage requirement for whatever specs were on the Tweaktown configuration?  Usually, to have 37A, you need to look above 550W.  650W sounds about right according to their review, that would give you at least 45A total and therefore room to breathe and not running peak.