This is why I absolutely hate pre-built computers and would advise anyone against getting one from these 'big name' cheapskate brands (HP, Dell etc...), that PSU to be blunt (and absolutely no offense to you) is absolute shit but that is enough wattage for the GT 730, hopefully that PSU does provide enough wattage for all of your components and the GPU.
That card will be absolute fine with Windows 7 when DirectX 12 comes out