Will my gtx 750ti run in 250watt?

Admittedly, Nvidia specs the power needed a bit higher than what is used by the card on average. However, the graphics card will at times peak higher than expected, so you do need a little headroom. That being said, you can't forget about the rest of your components (some CPUs can use even up to 150W), so make sure to plan for the proper PSU size appropriately.