Yes, there are a few things going on. The i5 2400 will bottleneck your GPU even at 1080p. You have 8gbs of ram, so for those games you mentioned at 1080p you will be short (as most use upwards of 9gbs system ram at 1080p).
Neither 2400/3340 mentioned will help any, as snipergod87 pointed out. The issue is that hey are 4c/4t CPU's which are just hitting their max right now. You need more cores/threads for smoother gameplay. When your CPU or even the 3340 hit 100% usage, which they will in nearly all of those games, it can cause massive stuttering in game. Not nice.
You really need to do a system upgrade to get the benefit of what is a really good GPU.