If you want to hear minute details you need bigger speakers. Computer speakers are not going to do it for you.
What is your budget? On a smaller budget I would probably recommend a 4.0 or 4.1 system. Sounds like you need either powered monitors or passive speakers and a receiver.
A sound card may or more not be necessary depending on what system you plan to get. Once you determine what speakers you are going to get, you can work out if you need a sound card.