You're going to need at least a 300w PSU to be able to run the 750ti reliably. The reason that that low of a PSU is in your prebuilt is because it only supplies enough power to run those components as they should. In conclusion, no you can not run the 750ti. The 750ti takes a maximum of 68w which is more than half your PSU. I recommend upgrading to a 300w PSU before upgrading to that GPU. You can find some nice bronze certified ones for around 40-50$.