For low wattage cpu systems, absolute minimum for a gtx750ti is 300w. For your i7, 400w is recommended. The minimum for a gtx950 is 380w,or basically a 400w psu. So for either card, the 400w would be fine. If it was a good psu. It's not. HiPro makes a majority of the OEM psus for HP, Dell etc and is considered a direct replacement. It's not a gaming class psu by any means, and thats what it takes to run any decent gpu. Those OEM psus are designed to run for years with no upgrades on a basic pc, which is why there are gaming psus to begin with. OEM's just can't handle the high stress demands of a dedicated gpu for very long.
There are always sales on XFX, Antec HCG and a few other very good gaming psus, some of which can be as low as...