badhomaks :
For some reason I could not find any stores in Canada that have the 450W version in stock. So is it ok if I go with the 750W version? With that much watts I can add another 670 in the future with no problems.
With the 450W, you are NOT OKAY to do SLI with 2 GTX670s. With the 750W, you ARE OKAY with SLI, and then some. That is a factor for you based on your desires to SLI.
And looniam is right, it is reasonable but not the best to assume that the review of the 450W unit extrapolates and extends to the 750W unit, for one, I suspect the topology is different because the 750W unit is much higher powered, and USUALLY the topology changes for PSUs if you want efficiency at above 500 Watts or so.
One shouldn't skimp on the quality (design, engineering, component selection, testing) of a PSU, which is not the same as the wattage.