The power draw of your system is exceptionally low, you have a processor that will not draw more than 50w, and the GTX 750ti runs directly off the motherboard PCI-E slots, being restricted to a MAXIMUM of 75w.
Even on a no-name PSU, Your main components are already only at 125w. That is why the 750Ti is great as an upgrade for older/prebuilt PC's. Its power draw is so efficient you are essentially able to simply drop it into a PC without issue.
---Your 400w older PSU should not pose any issues with the 750Ti, even if it is a 3 year old OEM PSU.