I'm not sure why you've have disabled crossfire since it will perform better knowing how to utilize two graphics card rather than one which you might have accidently have setup up? By having crossfire disabled even though two graphics card are physically connected to the motherboard as mentioned before won't be fully used. I suggest either you upgrade your current graphics card if you want one robust card or enable crossfire again. Separately here is the link to the wow system requirements.
World of warcraft ingame has it's own graphics recommended configuration, i suggest you use that rather than adjust it yourself unless you feel comfortable doing so.