Bang for the Buck

I am gearing up for my first build. I am on a budget, but still want to be able to run my games on high graphics without noticeable lag. I play games like Sims3, Fallout, SWTOR, Warhammer 40k, etc. I am currently planning on getting the ASUS P8Z68 Deluxe for a Mobo, and the Intel i5-2550k for a CPU. I was looking into the EVGA GTX580 for a GPU, but it is a little pricey. I want to stay with a single GPU configuration. Is there anything cheaper that will still perform in the realm I am looking for? Thank you for your input.