If you're only playing at, or near, a resolution like 1920x1080, then any mid-to-top-tier GPU made in the last several years should be more than enough, and if you plan on turning up your settings, then the more VRAM, the better. Also, if you ever plan on dabbling in BFA with more than two accounts, then the current-day graphics require a good bit more juice than what Classic uses, so it might be worth considering a slightly more powerful GPU if the price difference is small and you can make the jump (you will also want 32GB of RAM for comfortably 5-boxing Retail, if it comes to that).