It will normally have the greatest impact where a video card is waiting on the CPU. In that case, when you free up CPU cycles, the video card is able to do more.
However, if the video card is already saturated, then it will not make any difference in the frame rates, but it will help in other areas, such as Internet connection handling.
Bottomline, anything you do to free up CPU cycles is a good thing.
The other part of the sound card equation is the driver. If you simply disable the onboard sound and install a new card, you will not gain much and could possibly cause instabilities in the your system. You must remove the drivers for the old sound card first.