wondering which parts i should upgrade

[Unobtainable]

Hello
Im thinking of upgrading my computer a bit, but im not sure which parts i should replace, to get the most bang for my buck.

Asus P7H57D-V/EVO
Intel i7 870
Kingston HyperX 8GB DDR3, 1600Mhz
Sapphire Radeon R9 280 3GB
XFX ProSeries Core Edition 550W
OCZ ARC 100 series SSD - 240GB
Seagate Barracuda 7200.14 - 1TB

Im thinking Motherboard, CPU and RAM, but would like your take on it.
Thanks.

 

[ComputerSecurityGuy]

CPU/motherboard. Xeon E3-1231v3 if you want to keep the RAM, i5-6500 if you don't.