It depends what onboard chip you have. There could either be a really big difference in sound quality or not a whole lot unless you have a well trained ear.
Where Creative sound cards really stand out is in 3D game sounds. Although I bought my Audigy 4 because it did a lot better for just stereo music than the onboard I had on my old setup.
It does help boost performance slightly since it does the audio proccessing instead of handing some of it off to the CPU. I would think it would help most in games like BF2 since there are so many sounds at once (I'm guessing).
All on board sound is inferior to a real soundcard. Disabling sound in games will give you a performance boost... Or to put it into perspective, using onboard sound will have a negative impact on performance.
Get a sound blaster audigy (like me) if you want a really cheap one that supports EAX 2.0 and HD audio (in games like PREY and FEAR)
The 3dmark03 benchmark showed a 1fps difference on the audio benchmark.
I would recommend the X-fi Music, or sound blaster audigy 4 though if you are prepared to spend. Whatever you do DONT use onboard, the sound quality is never as good as well as performance.
For the avarage user, no it isn't. Onboard sound has been really acceptable since a year or 2. AC97 is pretty good. Though anyone who enjoys any form of 'sound' should get a soundcard and a nice pair of speakers.
eax on the ac 97 was done horriblly. with a sound card the sounds are much crisper and doesnt sound bogged down. plus in games for example like battlefield 2 you are able to hear more sounds.
some people say that having a sound card is like night and day - others say there isn't much of a difference....I think a lot of depends on how good your ear is.
Just like how some people are perfectly happy with a 15in LCD at 1024x768 while others swear by a 20.1 inch widescreen at a resolution of 1680x1050.
this is also a huge factor - if your going to shell out for a nice sound card you might as well make sure you have a nice set of speakers or headphones.
No sense of having a Creative X-Fi playing through a set of junky $20 dell computer speakers.