What monitor do you have, it's resolution and refresh rate ? CS:GO is pretty easy to run, particularly if you play it to win and turn settings down to low to improve your game play.
Max details is a different story, you can go 750Ti for low details. For High and new maps like de_nuke I would not go lower than GTX780/GTX960/HD7970/280x/380x.
Actually on de_nuke I get over 80% GPU utilisation on 1920x1080 max details 4xMSAA on i5-4460 and R9 Fury.
http://imgur.com/ADriChG
http://imgur.com/8yz9HBj
So even with GTX970/R9 290 I could be GPU limited here.