Radeon r7 240



As octaneurpc said the R7 240 will work. However you might want to consider a different card. The 240 is pretty basic, there are much faster cards available for very little difference in price.

Based on the details of your system you may be limited by the psu so probably best to stick to a card that is powered by the PCIE slot itself. The R7 250 or the HD 7750 are both good options. The *fastest* slot powered card available is the new Nvidia Geforce GTX 750ti, although that is quite a bit more expensive.