It will run it fine on lower settings and not a high resolution...
I have a 560Ti that I am running at 1920x1080 with everything on ultra with 8x filtering that starts to choke in 25mans. So you will not be able to get anywhere near that, but turn your settings to low, and drop the resolution and you should be fine.
Looks like that GT120 is an OEM MAC part correct? If you have this card and can upgrade the card, do it. It looks like it's selling for $150 on Apples site, which is at least $100 more than what it's performace is worth. If you are about to buy it, don't. It's a 2009 part, based on a mainstream video chip, I would not buy it used for more than $25 or 30. A new $70 card will be 3-4 times faster than this.
I have pretty much 0 experience with MACs but something has to be better than this for one. It will run WoW, maybe even at decent frame-rates at lower settings. But is really really really not worth the 150 Apple is charging.