I’d agree that you should get a new GPU. I don’t think by changing the Mobo/CPU you’re going to see $300 worth of improvement, however I do think you’ll see a lot more improvement if you get the best GPU for $300. Then next time change up for an intel CPU/Mobo. That’s what I would do.
Yea, if you want to upgrade your system, I would buy a new gpu and upgrade your ram to 16gb. A new CPU would most probably end up in buying a new mainboard. If you don’t have a ssd, you should think about buying one.
Upgrade your GPU, I have an AMD FX6300 CPU, and upgrading to a Radeon R9 290 really improved my UE4 performance when I replaced my old well, not so old HD7790. Also, Look into doubling your RAM if you can, that might be a nice idea too.