i am assuming this pc will have a low watt power supply to go with it's age. so a low power card is what you need to go with it. the r7-240 is very low watt power at 30 watts and will offer the hd playback your looking for. this link goes to a few cards that are in this family and cost between $55-$65 http://pcpartpicker.com/parts/compare/asus-video-card-r72402gd3l%2Cgigabyte-video-card-gvr724oc2girev20%2Csapphire-video-card-100369lp%2Cxfx-video-card-r7240aclf2/
if you actually have a new psu with over 300 watts, you can get a better card that will even allow for decent gaming. they will run over $100 though and not worth it if the system is only for video playback.