Yes, the 400W/450W PSU can handle your PC with gtx750ti, but if you can find the 500W/600W one for less, just buy it, why not? but make sure buy the good brand PSU, eg. XFX, Seasonic, some of the EVGA/CORSAIR, etc.
Also if you want to upgrade the graphics card I have the same. Computer. I managed to fit a msi ventus rtx 2080 but had to unplug a wire from the motherboard. It's a tight fit but it will. Also the wire controls the beeping noise during boot up. So don't be scared to unplug it