With only a 180W PSU, you will be limited to a card that doesn't require a separate PCIe power connection; i.e. one that gets all its power from the PCIe X16 slot (75W).
One of these HD 7750s would be the max you can go. The HD 7750 maxes out at 55W.http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20600286767%20600298541&IsNodeId=1&name=Radeon%20HD%207750
You could also go with an Nvidia card, but they have nothing that can match it that doesn't require a power cable.