If it's a live 5.1, it will be better than any on-board audio I've heard. I have just about every one imaginable, and I've listed to most of them on my JBL Studio Monitors. The regular sb live is still good but like the previous poster said, surround is lacking on it. The live has a much better s/n ratio than most on-board audio except the envy if I recall, but the envy never sounded as good to me.
The live will also offload eax processing wheres on-boards may support it, but they don't offload it. You could see upto 5% cpu reduction depending on material and your system.
When it comes to newer cards, they are so very close, most people won't hear a difference. My audigy 2 and X-Fi both sound slightly better than the live with eax, course they support higher versions. Music is almost identical, the live is slightly less dynamic in my opinion. The audigy and X-Fi are so close, any difference is my mind playing tricks on me. You can play the X-Fi much louder without hearing ANY noise however.
Course this is just my opinion from listening to many chipsets. Everyones ears are different and you may think the live sounds bad compared to a alc861 or alc850
I also have a Diamond Xtreme Sound 7.1 that uses the CMI 8768 chipset. It sounds as good as the live, but not quite up to the audigy and X-Fi. A great cheap card though.
I will say in my opinion CMI based chipsets tend to sound better than Realtek, and the Envy falls in the middle when comparing the higher end CMI and Realteks. Now if you can find an MB with a good CMI chip, it will sound better than most integrated ones, but being integrated on the MB will always cause a higher noise floor and thus more noticable noise.