Not all high end headphones need a dedicated amplifier, but there are plenty of headphones that benefit from the extra juice a headphone amp provides, even though, they may work just fine with standard line out.
One could either put a headphone amp between the phones and the sound card, as Dup said, or get a DAC with headphone amp, or get a separate DAC and headphone amp.
Depends how much one is willing to spend, and if their sound card works fine, with no interference issues. If one has interference issues, adding an external headphone amp is pointless. It would be better to buy an external DAC with optical connection, as that will not pick up on any interference produced within the PC case. Plenty of DAC's have USB, but USB can be prone to interference.