Not knowing the brand of your power supply I will recommend mid range cards.
If you have a good power supply then you can use higher end video cards,
If you post what brand and model. And a budget it would help.
At a resolution of 1366x768 a GTX660 would run high to max on most games. around $200
A GTX650TI Boost 2gb or GTX 750 TI would give you medium with a few high settings. around $150