A sound card is only used these days by folks who work professionally with audio or whom have very high quality speaker configurations (audiophiles?). They work great for eliminating the negligible amounts of background noise that is sometimes generated on integrated audio chips.
Most integrated audio is seriously good enough for most people, myself included. And if you only plan on gaming or using the machine as a general media box integrated will be fine.
Unless you feel the odd need to fill all your PCI slots or if your motherboard doesn't already support your desired outputs (2.1/5.1/7.1...) I wouldn't recommend spending the money on a sound card.
I use a sound card, and i can say it's way far better than a onboard audio (i have THX certified audio on my motherboard and it's crap vs a real sound card, i had a X-Fi Xtreme that worth 20$ and it's better so, i now have upgraded for a Xonar DX and WOW !!!)
The sounds effects are better, the positional audio is better, the sound management is better, and the lack of interference with a dedicated soundcard is simply awesome...
PS: you'll see the difference more with a good headset/headphones than on speakers as most mid to high range speakers have a internal equalizer like the logitech X and Z series... (i use a X-530 5.1 setup and Steelseries Siberia V-2)
On the Headset, no sound from mice interference or cellphones that are aside the speakers, higher volume in headset (greater voltage output) better quality in high volume (no distortion, clear sound) The quality is simply here over an integrated audio chip... However, if you'd never try that, you'll never see that onboard is crap (i'll never return to onboard, i tried the THX onboard to not have a dedicated sound card and no, real crap like other ones and it's almost the better onboard chip 24bit 192khz but only 88dba SNR over the 106 of the Cheap X-FI and the 116 of my Xonar)
SNR is the sound to noise ratio, higher = less interference cleaner signal so better quality in sound and each 5db is the double of sound quality (in theory) the xonar also output 2Vrms up to 600ohm headphonnes over the 1.1V and 32ohms of the integrated chip and the 1.3V 60ohms (standard for a sound card) of the x-fi...