What speakers exactly do you have? And what motherboard?
It's pointless spending much money on a decent soundcard if you dont have good enough speakers to utilise the card, and in most cases the on-board sound will be nearly as good as buying a cheap sound card.
I'd find it hard to believe your motherboard doesnt support 5.1 audio - unless it's 10 years old, so if that's all you're buying for then maybe rethink