I've heard that having a dedicated sound card takes some load off the processor which would mean better performance overall. I'm not really a sound-freak nor a hardcore gamer but I just dont want to add something that detracts from the performance.
My current system:
Intel Core i5 760 @ 2.83Ghz (everything stock settings )
430W Antec Basiq Power Supply
XFX Radeon 5670 1GB
500GB Seagate Baracuda
Windows 7 Professional
^+1 i agree 100% if you don't have high quality speakers or headphones a soundcard is a waste.
If you have quality speakers/headphones, yes , a soundcard is a improvement over onboard.
Do you have to spend alot of money?
I have a Msi P55A-GD55(secondary rig)with an Auzentech X-Plosion ($45 CAD) hooked up to Logitech X-540($50) and it sounds good to me.
I realize neither of the above components are first rate but honestly for the money it's a major improvement over onboard and cheapo(generic) speakers.
Both prices were basically half price on sale.
Quality sound on a budget can be done.
I do alot of recording (guitar,keyboard,vocals) on my tower so a soundcard is a must.
Though I went with a Xonar DG to keep costs down.
24bit/48khz sampling is fine for the recording I do.
Also use a Creative surround sound speaker system so I think it makes a difference there also.