OP Like many others have already said your best upgrade here is going to be the CPU, AMD just doesnt even compare to intel when it comes to single threaded performance and since ED have said they wont be supporting multithreading with edge its a no brainer.
Even in the current engine you will see a performance increase, as i used to have an almost identical build to you with an 8320 OC to 4.5ghz and 280x, I upgraded to a 4690k and even at stock performance almost doubled in DCS (most apparent at busy airfields and heavy combat areas).
After the CPU upgrade and the release of edge then I would start thinking about a possible GPU upgrade but the 280x is still a decent card.
Things like an SSD and extra ram are great but should be afterthoughts to a CPU or GPU upgrade.