I would say a nVidia 670GTX or 770GTX is a pretty inexpensive way to go, and won't cost you too much. 670GTX will prob run you $350, and the 770GTX is just over $400. Both will game fine, and should be good for any editing that you though at it.
Don't buy the 670 just yet. The newest GTX 760 *should* work in OS X and has comparable performance to the GTX 670. The 760 has just been released, so hopefully we will see some people try it out within a week or so.
Maybe wait until the 760, it should be a card to well suit your needs. Although the GTX 770 is an absolute beast, and it will certainly future proof you, especially if you decided to add another monitor.
If you do choose one, the Asus brand cards (with Direct Cu2 or something) of the best cooling and longevity solutions in my opinion.