Best upgrade and keep cost down get a better GPU maybe RX480 or GTX 1060 depending on your budget
But if you want a decent new PC for just WOW you could look at the following:
Intel Skylake Core i5-6600, 3.30GHz, Quad Core, LGA1151
CORSAIR Vengeance Pro 16GB (2 x 8GB) 240-Pin DDR3 SDRAM DDR3L 1866 (PC3L 14900) Memory
GIGABYTE H170-Gaming 3 DDR3, Intel H170 Chipset, LGA1151
GIGABYTE GeForce GTX 1060 WindForce OC GV-N1060WF2OC-6GD, 6GB
It will cost you around $642.30 on Newegg for all that