In my opinion i Would upgrade the PSU to something newer that actually has a PCI-E 6pin but you can always resolve a PSU without the PCI-E 6 pin port with a Adapter LP4-6PIN For example
https://www.amazon.co.uk/StarTech-Express-Video-Power-Adapter/dp/B0007RXDDM/ref=sr_1_1?s=computers&ie=UTF8&qid=1475083096&sr=1-1&keywords=lp4+to+6pin
but if he doesnt want to get an adapter the 750ti would be the way to go since the tdp is low ,And if he does want to upgrade his PSU Than i would Recommend A XFX 650W Bronze+ PSU - It could be overkill but its still a good price at around £50-£55, But your Idea Is right Of the 750TI Thats def the way to go thats what I would recommend but he has a proprietary mobo so only that PSU will have the right cables for his mobo but alll he will play is Overwatch, csgo and MOBAS I have a 750ti Windforce and It run bf1 high-ultra @1080P with 60FPS. I think the 750ti is perfect for him but I am just worried about if there is enough Watts
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