With a slim line case and a small 250W PSU, you are not going to be able to get a very powerful gfx card... and it will have to be a low-profile design. Is this what your PC looks like? http://cnet4.cbsistatic.com/hub/i/r/2010/08/03/59187482-67c4-11e3-a665-14feb5ca9861/thumbnail/770x433/e61e3d336097d52ab2256c1fa7f66dac/33345348-2-440-FT+BK-2.jpg
Especially the back.
If so, the best you can do is one of these R7-240 cards: http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20600038363%20600038364%20600001667%20600473872&IsNodeId=1&name=Radeon%20R7%20240&Order=RATING&Pagesize=20
They come with the full height bracket attached and a half-height included you will have to swap out. Be aware that the last card in the list takes up 2 slots. So check to see if all is clear inside the case. And measure the room inside the case from the expansion slot covers to the first obstruction the card would run into (lengthwise).
They should be able to handle those games with that processor at lower resolutions; 1280x720, 1366x768, 1440x900
If you have a 1080p monitor, it will struggle. So a lot depends on the resolution of your monitor.