what astralite said. your greatest benefits from the addition of a sound card would come to you if you had more 'active' needs either in terms of working with external source material - specially if it's analog - or if you wanted to venture into building your own cheap home studio to record music. pro-audio sound cards offer expanded inputs / outputs, digital/analog converters and basically the capability to do more with external sources. However, these days the sample rates that were once the dominion of high end sound cards are pretty common on 21st century motherboards.
i'd think through what you really want to achieve before jumping into anything. even if, for instance, you started making recordings and wanted to get those onto your hard drive to edit, etc., most of the new portable audio recorders - essentially replacements of DAT recorders and minidiscs - rely on the same sd card storage technology you use in a camera - and this includes audio industry mainstays like tascam as well - so it's hard for me to recommend a card to you even if your uses were more more highly specialized.
as astralite suggested, concentrate on other components if all that you're really after is better listening experience. it's a great time for budget audio gear -much to the chagrin of those of us who have spent small fortunes in our lifetime - and really fine sound can be achieved on a modest budget. the first thing i always did was throw out the speakers that came with pc systems back in the day. times have changed.