If you will be playing just one game exclusively, then look for benchmarks of that game at your resolution.
Past that, few games use more than 2 or 3 cores, making the i3-2100 an excellent gamer, and a quad will not be that much better.
My rule of thumb is to allocate twice your cpu budget to the graphics card.
Read this article on <$200 gaming cpu's:http://www.tomshardware.com/reviews/gaming-fx-pentium-apu-benchmark,3120.html
The tests were done with a strong graphics card, a 7970.