The difference between onboard and dedicated SC is narrower than it was years ago but the difference is still night and day. Why anyone would pay hundreds of dollars/pounds for a new rig and then use onboard SC is beyond me. "It's good enough" is the usual reply, but anyone who has tried a good SC compared to onboard usually never goes back to onboard.
I am using a Xonar DX, the positional audio and range of sounds from it compared to onboard is amazing. See this review from a few years ag o for the Xonar DX, the listening tests mostly apply to music but I found the same for games. For example in my tests on Rise of Flight the onboard sound was tinny and lacked range compared to the Xonar DX.
http://techreport.com/articles.x/14500/6