Hi....whenever you ask a question, or look for advice it's always a good idea to detail your computer's specs, it will save you a lot of time. Making a sig is a good idea, so you only have to do it once.
The GTX 460 is an amazing card, and very inexpensive now. In fact, if your motherboard, and PSU can handle it, a pair of them in SLI scale beautifully at around 90% in most apps, and will outperform a single GTX 580. A 650watt PSU is sufficient, but 700watt+ would be better.
I occasionally see these on sale for around $159.00
HAF 932 Advanced / Corsair AX1200 / Asus Rampage IV Extreme / i7-3930K / H100 Cooling 4x120mm
16GB Corsair DDR3 / GTX 580x2 SLI + GTX 460 PhysX / SSDs Mushkin 240GB, Crucial 256GB, Crucial 512GB