For a single 1920x1080p monitor, 1GB of VRAM is sufficient unless you crank up aliasing or sampling. 1.5GB's will be plenty, especially for a non intensive game like WoW.
And I agree with Nsouter, there are better cards at the GTX580's price point. A GTX580 go's for ~$420 on Newegg, which is exactly the price point for a GTX670. They perform better, can overclock past GTX680 performance, greater energy efficiency and has many more features. Comes with 2GB of VRAM as well.
Gigabyte GTX670. $400http://www.newegg.com/Product/Product.aspx?Item=N82E16814125423