There is a lot of crontraversy around this topic.
Bassically it's what pizza said.
Consider it like a car engine. The sound card being the fuel pipes and the engine being the speakers. The fuel then is your sound. If you have bad sound quality songs/audio files(too little fuel: fuel being the amount of kbps of the audio file) to begin with then the rest of your audio setup doesn't matter. If you do have very good quality songs like 320kbps, but the fuel pipes can only transport half of that quality/fuel to the engine/speakers then that quality goes lost due to a bad sound card. This doesn't really matter if the engine sucks too because well good sound quality files aren't say expensive. If you're engine is great however and the fuel pipes are too thin then you're losing a lot of money. The other way around is the same if your sound card is too good for your speakers.
All in all there has to be a balance between speakers and sound card. The sound files just have to be as good as they can be.
So the question is what sound system do you have?
Also for gaming the sound will also be better and sound cards have virtual surround software making positional audio in games better.