res doesn't matter cuz ur shit3 is bottlenecking, not gpu
I've got a GTX 950 myself, and the biggest FPS boost I got was when I downgraded the NVIDIA driver to version 378.49 - gained me around 30-50FPS in csgo. On the other hand, most other games do now run way worse.
Worth a try, maybe for your graphics card this isn't the best driver version, but maybe you can test it yourself.
If you want to try it, use Display Driver Uninstaller first.
the lower the resolution is the more u utilize ur cpu
but since 750ti isnt a great gpu there isnt much difference, mby u should get at least 960 even tho csgo is cpu dependent and u wont benefit so much from new gpu
i have i5 3570 + GTX 570OC, 1024x768, fps over 200 mostly, drops to 180ish in rush, or on heavy maps.
Change CPU and you will gain fps boost, GTX750Ti is totally fine for CSGO.
Just to point, i5 3570 costs 50€ used one.