Since u have only a 300W PSU... Consider a HD 7750 GDDR5? There is no need to have a 6-pin power conductor for her, your PCI-E port could support.
HIS iCooler H775F1GD Radeon HD 7750 1GB 128-bit GDDR5, $90
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161403,
it's the fastest GPU which doesn't need additional power supply I could find.