Yeah your bro needs a new PSU imo. Even with a 750ti your going to be on the edge and that things is 12 years old you said.
So its essentially degraded over the years and maxing it out may cause it to blow immediately or soon after adding the GPU.

/edit
something like this would work,

https://ca.pcpartpicker.com/list/PYPwq4

This post was edited by King Atrhur on Apr 28 2019 07:08pm