Which 5.1 speakers do you have?
And I do recommend buying a dedicated soundcard, however, as my music technology lecturer at university told me... "You have to be prepared to pay for it - the cheaper soundcards barely surpass the on-board sound chip.
Now granted, I've only had one soundcard in the 9 years I've been able to afford to build my own PC and that's been the Creative X-Fi Fata1ity which has been great. However, as motherboards start to curtail PCI slots in favour of PCIe, that will no longer work in some.
Creative's support is also fairly poor. Forum posts/posts to the company often are met with no/little response.
Their driver update program is pretty shoddy too. BUT, their hardware is great.
The other big player of mid to high-end audio (I'm not talking going into the professional range) is ASUS.
Whilst this particular sound card is expensive and depending on your speakers, might not be suitable, the video is really interesting that Newegg provide:
http://www.newegg.com/Product/Product.aspx?Item=9SIA24G3KG4914
Ultimately, it depends on a number of things.
1) What do you want your sound for? Gaming or audio creation?
2) What speakers do you have? You say you have 5.1, but if it's a cheap 5.1 product, it's not worth paying mega bucks for a decent soundcard.
3) If you're looking at replacing your speakers, which way are you going to go? A pair of monitors, or stick with your 5.1?
Pros and cons to both - the monitors will sound better, but they're going to be bigger/bulkier, they're going to be more expensive and you'll only have stereo sound. Surround sound - you generally only have tweeters/mid range and then all the bass comes from one source.
Hope this proves food for thought but also not too overwhelming!
NC