The GPU would be a mandatory upgrade, it’s well below the minimum requirement. The CPU is enough to run the engine, but it wouldn’t be pleasant once you started creating more complex levels, it would probably need an upgrade too if you’re serious about developing in UE4. You can wait and see though, the GPU is in greater need of replacement. You have enough RAM, as long as you aren’t running a bunch of programs at the same time. Down the line you might want more, but it isn’t a necessity.
Your CPU has a locked multiplier, which means that your only option for overclocking is through the FSB, which only gets you small gains, not a viable alternative compared to upgrading. I don’t know off the top of my head, but I would assume that your motherboard is only a 3+1 phase and not designed to handle the extra current a heavy OC would draw anyways. Epic lists an nVidia GTX 470 as the min GPU right now, you can run the editor with a less powerful card, but I wouldn’t recommend it since you sound like you want to create some graphically impressive scenes.