NVidia GTX 660 or GTX 660ti, AMD Radeon HD 7850 or Radeon HD 7870. However, you will need to find out if your power supply has the required PCIe power cables. You will need one PCIe 6-pin connector for the GTX 660 and Radeon HD 7850. You will need two 6 pin PCIe power cables for the GTX 660ti or the Radeon HD 7870.
If your power supply is a stock supply in a prebuilt computer, it may not have any PCIe power cables so I would check it out first. In that case the best you can get would be a Radeon HD 7750 which doesn't require any PCIe power cables.