You'll be lagging bigtime for sure when it comes to large raids or densely populated zones and cities if you're playing even on a 20" monitor even on medium settings. I would strongly recommend a better graphics card regardless.
If you're on a budget, a 5770/GTX 460 should be alright.
the best you can go with is 1280x768...and in this case wow will run perfectly on your system....
A 15" monitor should be fine then, but I would still look into the possibility of getting a GPU upgrade if you can get one for a good price should you decide to play everything on high settings