Sound card worth it?

i searched the forums couldn't find the question im looking for.
i have the tritton ax pros. i got them for gaming on the pc and ps3. on the ps3 they rock. for games and movies. but for the pc it's a let down. still good sound but dosent sound like 5.1 like the ps3. i got onboard audio evga 780i.
is it worth it to go with a sound card? i use the optical conection, and yes ive done the +10 setting. i read that it wont matter optical is optical i also have 2.1 logitech speakers dount it will make a difference with a $25 set though.