I ran a quick test on CS:GO to see how much CPU it was using. Turned out to be more than I expected, regularly getting 60-80% on an i5, sometimes in excess of 80%.
This screen cap is taken on my system with an i7 4790K hyperthreading off (aka "i5 mode"), using a GTX970 at 1080p. It was taken during online deathmatch. With hyperthreading on it was similar usage, but only using the 4 real threads makes it easier to see how much the CPU is being used.
If you're looking for a CPU and can afford an i5, it would seem like a quad-core CPU is useful even for an "old" game like CS:GO.
Interestingly though, when running with 2 cores + hyperthreading activated, the usage didn't go up by a whole lot:
This cap is with 2 cores and hyperthreading activated — essentially an i3†. It gets into the 80-90% more regularly, but it's not that much more. I can only guess the engine is a bit clever in scaling what it does based on the resources available. I did notice a few frame drops into the low 100s, but the gameplay didn't feel particularly different.
† The i7 has a higher base clock speed and a much bigger CPU cache, so it's not a true test for how an i3 would behave