upgradeable build for gaming

[Zackery Underdown]

I just bought a new system meant to be a future proof gaming pc. By future proof I don't mean I payed an absurd amount for over the top parts but rather left a fair amount of room for upgrading later on. What i want to know is what should i upgrade first?
Here's the build.

Motherboard: Asus Z87-K

Case: Cooler master storm stryker

CPU: i7-4770K

GPU: Sapphire R9 280X

Ram: Corsair vengeance 1866mhz 8gb(2x4)

CPU Cooling: Cooler master Seidon 240m

Hard drive: A-data premier pro sp900 128gb SSD (1tb external HDD)

Optical drive: LG BD-ROM

Power: 850W Cooler master silent pro m2

 

[Cons29]

do i understand you right, you just bought this and you want to upgrade it now?
the build is good (though i would not pick CM for a PSU)

if you have extra, i'd get a mechanical keyboard with a wide mouse mat
or a good 27 inch monitor if dont have one yet

 

[firo40]

Crossfire another r9 280x

 

[Novuake]

do i understand you right, you just bought this and you want to upgrade it now?
the build is good (though i would not pick CM for a PSU)

if you have extra, i'd get a mechanical keyboard with a wide mouse mat
or a good 27 inch monitor if dont have one yet

The CM m2 Pro's are good PSUs. Very good actually if you consider the cost.

 

[Cons29]

that is just a personal opinion
with lots of choices out there, i'd get a seasonic or something

 

[Zackery Underdown]

do i understand you right, you just bought this and you want to upgrade it now?
the build is good (though i would not pick CM for a PSU)

if you have extra, i'd get a mechanical keyboard with a wide mouse mat
or a good 27 inch monitor if dont have one yet

I bought it in this state because of the cost I plan on upgrading to a more finalized build over the next 3-4 months. ultimately i want to ramp it up 16gb ram@1866mhz+ another R9 280x and also either another(larger) SSD or a 2tb HDD my question was more aimed at where do i start since i already have an idea of where im going.although suggestions for things i might have missed are more than welcome.