A better question is what games will my dual-core CPU struggle to play?
Generally, as long as you don't spend more than 150% of what your CPU is worth, you won't have trouble with it holding back that GPU. Based on this, you're probably looking at an HD 7850 or GTX 660 being the least expensive cards where you'll start to see bottlenecking.
I'd suggest a GTX 650Ti or HD 7770 unless you're planning on a platform upgrade soon.