apple products generally were known to have fairly powerful amplifiers included in them compared to other devices in their class. it could be that your onboard sound on the workstation is just very poor. of course now apple is removing headphone jacks from devices but that is besides the point...
yes, a soundcard will likely help. it would certainly be nice to know what headphones you are using in case they were something a headphone amp is more useful to power but i will list a few soundcards below...
on a budget... the xonar dg. pretty decent amplifier for a cheap card.
on a mid tier budget... the creative z. rather strong amplifier for a soundcard which can handle even some hifi headphones decently
on a mid tier budget... the xonar dx. better dac than the z but lacks a very strong amp. good enough for easy to power headphones
if you only need stereo sound, you also could go with an external..
the fiio e10k is a usb dac+amp that will handle many headphones
the fiio a3 is an amplifier alone (uses your onboard to process but just amps up the signal) which is also a good choice for headphones.
my suggestions (pending you listing your headphones) would be the e10k or creative z. if you needed an external solution only... then perhaps the creative omni or e10k.