50 ohms is pretty modest impedence in the grand scheme of things, so they probably don't need/benefit from a headphone amp as much as some headphones. I'm sure there will still be an improvement though.
Most headphone amps have phono inputs, so you'd need a 3.5mm stereo to twin phono lead and it would plug into either the headphone socket on your speakers or preferably the front speaker output on your soundcard.
In the meanwhile, it might be better to connect your headphones straight into your soundcard rather than through your speakers since the wires and connections in those speakers will probably be quite low quality.