You could aim for a gtx 1050 (in a few days will be available) and definetly the ram will help, need to find out which motherboard you have to find out which ram to aim for.
The 300 watt power supply limits you, in the way of gpu needing to be pretty low end. Because that hp build probably has one of the cheapest psu available. In the end you will still have a weak cpu, ii think wow is more cpu intensive. That 1050... It may be worth going with an rx460 to pair better with cpu.
End result is... Your cpu is weak for your game.
You could take a path of i5-4460. Gtx 1050, 8gb ram, motherboard,, 500w psu. Maybe a case would be an idea. This is the cheaper build which will get you further.
Let me know what you think of this