Your CPU is most likely the bottleneck because Source is a CPU-heavy engine, but you're not going to get much better. You need to look for single-thread performance, so your best bet there is getting a K-series processor to overclock, but it seems pretty wasteful when a 4590 is still a pretty good CPU.
I think you might still want to consider upgrading your GPU if you play any non-source games. Outside of CS/TF2 upgrading your processor is going to do next to nothing as long as you still have a 750Ti.