at a gaming and general audio level you would not notice the difference.
its only as you get up into the profesional realm where it makes a difference.
It used to be by getting a sound card you could save the CPU from doing a heap of work but with CPU's now so powerfull you wont see a difference.
Hugo is right. First, motherboard sound has come a long way with HDA. Second, with all the multi-core CPU's, the CPU utilization rates involved really do not matter as much as they did in the single core days.
Soundcards offer several advantages, ranging from better signal quality, to a (significantly) better Dynamic Range, not to mention other audio enhancments that onboard audio does not offer [Virtual Headphone/Speaker, etc].
Anyone who says soundcards don't make a difference is deaf. To me, not using a soundcard is like playing a game with 0x AA. Its an enhancement, not a necessity. [Yet some people here would glady pay upwards of $500 to move AA up one more notch...]