Well taking in consideration your list of specs you can try to use your existing RAM, PSU and disks BUT you will have to choose your CPU platform with care because with the newest Intel CPUs you have to buy DRR4 RAM and they are only retro-compatible with DDR3L (DDR3L is crap).
With that range I would spend the most on the GPU because developers (theorically) are delegating more tasks to the GPU and there are less CPU bound games today. Only few games, like GTA V or Metro, needs a potent CPU. You can see this video if you want to see some graphs and stuff like that: https://www.youtube.com/watch?v=K9EJNa0y524 .
Interestingly the video shows that in 4K resolutions the difference between CPUs is rather small because the GPU performance is used at full rather than in 1080P, maybe you can use this to your advantage and use Virtual Super Resolution or something like that while using an inexpensive CPU. Your actual phenom is really old, maybe you could use it if you haven't even overclocked it. It would be nice if you could buy a new one but that makes sticking to the budget difficult.
If you want the best value go for the 390x, just avoid ASUS (they are pretty lazy with their coolers). It is one of the best value cards available and it has pretty good numbers on the Steam VR benchmark and DX12 games, it gets dangerously close to the 980 TI. I've got a friend that has a FX-8320 (overclocked to 4.5 ghz) and a 290x (technically a downclocked 390x with half VRAM) and it got a pretty good score (7.6).
Fatslob says that you should compare the 390x/980TI but i would rather recommend turning that into a Fury(X)/980 Ti comparison. The 390x and the 980Ti are different tiers and their prices are very different. If you want the bang for your buck go for the 390x, it has good VR scores and it has fared really well in DX12 games. Only if you have the money jump to the Fury(X)/980 Ti tier. Maybe you should wait a little bit since Polaris and Pascal are comming soon, but that is your choice.
Also, the oculus recommendations are rather vague, and the utility they made just compares your hardware with a list, I recommend you checking the Steam VR benchmarks instead.
