If your main use for this computer is world of warcraft, you wont ever find a situation where you'd need more than 8gb of ram. If youre playing at a resolution higher than 1080p (or plan to use their really cool rendering scale option to scale resolution to 200%) then you'll want a card with probably 4gb of ram. Luckily most maimstream graphics cards have 4gb of ram. My suggestion? Buy a prebuilt ibuypower with something like an i3 and a 750ti/r7 260x. My radeon 7870, pentium g3258 @4ghz and 8gb of ram run wow at absolute maximum settings (outside of raids and battlegrounds) at 45+fps. In raids and bg's i put rendering scale back down to 100% (1080p) and lower settings from ultra to high, and msaa from 16x depth/16x color to 8xdepth and 8xcolor.
Hope this helps.
EDIT: A radeon 7870ghz is about equal to an r9 270. (Or maybe 270x, cant recall at the moment.)