Two independent bass channels is not pointless, in a bi-amped setup, you have stereo bass and it does make a difference. Lot's of us do not want our sound processed and summed into one channel. It does not sound the same as stereo bass. While some bass is "omni-directional", this is not and accurate description, nor an excuse to sum the bass to one channel. (In this case I think the OP is doing mono bass, but nonetheless it is worth mentioning.)
On-board sound can sound and is crappy, on my old HP it was OK, on my new Asrock, the sound is ****, so I had to get a sound card, this is on a MB that is 5 years newer. Onboard sound is not "passable these days", in my case, it was a step backwards.
"Someone truly serious about music would invest tons of money into headphones instead of speakers.", truly the most retarded thing I have read here so far. While headphones can sound very nice, and be used for reference in some cases, it is not "better". Headphones have their own set of problems with sound reproduction, specifically extreme channel separation that can cause headaches depending on the source you are listening too. A "live" presentation is more realistic in an open environment when some sound from the left can bleed into the right, this is how us humans locate things.
Don't believe me? Have a listen to an early stereo track like a Beatles album, the extreme channel separation in the headphones cause some people's brain to bleed, play the same track on an "open-air" setup and it sounds good, it sounds more like it was intended.
"Bass is hard to locate"- no it isn't, if I watch a movie where a truck goes thundering by from left to right, it sounds better when the bass comes from the left and fades out the right, not a hard concept to grasp.
A DAC vs a sound card is purely semantics for the most part, they are the same thing. Aside from a DAC usually being attached via USB which causes it's own issues with jitter etc.
I am 95% certain you will notice an improvement with a decent sound card. The only way you would not is if your MB had amazing sound and you bought a cheap sound card.
In my case the Asrock board comes with "ALC892", which was not as good as the "ALCXXX" that was on my older computer. Also this number does not tell all, but just a guideline.
I also agree with the OP about 5.1, 7.1, useless drivel, it rarely works how it is supposed to and most times it is heavily processed. Music is recorded in stereo, movies still sound great in stereo, the surround does not work on most TV stations. It is not worth the hassle in my opinion for the little amount of time that the surround works properly.
Simulated surround is terrible, again try to avoid any kind of post recording processing. Surround may have a useful place in gaming, but that's about it.