You dont need a pre amp just take the lead straight from the head phone socket on your system to the sound card on the Pc its worth having a dual plug at the head phone socket so you can plug them in at the same time. I use Nero Sound track to record on to the pc and you need to make sure the levels are set correctly
I got one from Mapins at 2m long, no problems does not have an effect on the quality> I got set up for less than a tenner from Mapins certainly better than spending 50 quid or 100 quid on those USB decks