I bought a pre build pc some months ago. It is an asus with:
Amd a10 6700
And i recently installed a:
Evga gtx 750 ti
The question is.. It is a good gaming pc? What would you change if not? (Don't mention billionaire cpu and gpu).
But, because you are limited by your CPU, you may experience 10-15fps less than the average amount. Lowering the settings or resolution can remedy this though.
As a side note, the 750ti can be used in many prebuilt computers or old computers because of the low watt requirement even if the PSU are bad.(it varies, but in general)
So make sure to give it some use elsewhere when you get a new GPU.