I assume you have done some reading about CPU/GPU bottlenecks on Tom's, but in case you haven't, here you go:http://www.tomshardware.com/reviews/game-performance-bottleneck,2737.htmlhttp://www.tomshardware.com/reviews/game-performance-bottleneck,2738.html
But more to the point, I think you would find the following two articles very helpful:http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106-5.htmlhttp://www.tomshardware.com/reviews/gaming-pc-overclocking-pc-building,3273.html
Looking at the first article, you can see that your CPU is in the same tier as the CPU used in this quarter/month's $500 gaming oriented build. They went with a GTX 560, and as you can see in the benchmarks, it runs games reasonably well. In my humble and honest opinion, I think that a graphics card could extend the longevity of your rig, so go for it if you have the money to make a significant upgrade!