I think that you should go for a sound card because they are heaps cheaper than most DAC's and are a decent improvement in surround sound over on-board solutions. Although I don't own a dedicated DAC, I do own an Asus Xonar DG and from my experience, it offers a wider sound stage and slightly clearer sound than on-board. It's heaps better for gaming
I would suggest an Asus Xonar DG (or DGX if you want PCI-express, same card otherwise) or maybe a Creative Audigy FX. They're both decent cards but if you want 5.1 speaker support, go for the next models up.
I wouldn't stress about CMSS vs Dolby (or the likes), I find it's good in some games but pretty rubbish in others. You can always look up on YouTube about which one you like better. Also, you will probably notice the headphone amp if your headphones are 32 ohms or greater because it helps separate the bass and treble.