It not possible to say for sure with the information you have provided but I do have concern. We need to address 3 things: PSU wattage, PCI-e x16 slot availability on the motherboard and space constrictions inside the computer.
Your PSU is only 265 watts and this might or might not be enough to run the system. Even if it will you will be running the PSU basically at its max. Add to that the fact that PSU's lose wattage capacity as they age and you could have a recipe for disaster. That is, blow up your PSU and it might take the rest of the computer with it. So even if you can use this PSU I would still replace it, you will see savings in power consumption, it will be quieter and your system will run more smoothly. You did not state what GPU you are currently using so I don't know if you are adding additional power requirements to the build.
The GTX 750 Ti is a 75W GPU and uses a PCI-e x16 connection so you must have a open slot on your motherboard. Now the 750 Ti gets all of its power from the PCI-e x16 slot so you don't have to worry about having the right external power connectors from the PSU but in some rare cases the mobo manufacturer has reduced the available wattage from the slot down from the standard 75W. They usually write this fact on the mobo just above or below the slot.
Last, you have to be concerned about the card physically fitting inside the computer, especially if you have a pre-built mini tower or Small Form Factor case. These things might require a single slot and/or low profile GPU. Without knowing the rest of the system specs it is difficult to assess this issue.