use the sound card, first its a creative, and if you do any gaming well thats mostly the only thing they do now... plus onboard sound uses more system resourses that you might need for something else... thats the advantage of having a processing unit just to process sound in the add-in sound card (at least in the good ones such as the one you have =)
creative blew it in vista drivers for my audigy platinum2 , understandably its an older card but my onboard is decent for most things. I think alot of people had issues with audigy drivers on vista m at least i read enough to decide that creative was less an audiophile than I was
Its up to you, since your sound card is a Creative SB X-Fi Platinum 7.1 Channels
it can pretty much cope to your current system and will provide you extra features
that you cannot find on build-in sound cards.
The card's audio processor with CMSS-3D technology is able to
remix your stereo audio into surround sound—even through stereo headphones.
And if you enjoy making music as much as listening to it, this sound card offers precise,
low-latency audio recording capabilities without taxing your CPU.
-- If you gonna migrate to Win 7, i think Microsoft decided to provide broad
support on legacy devices including those Sound Blaster's, so you're safe
using it if you're on Win 7. Not sure with Vista since it has many questionable
features particularly on drivers.
I used to have a Creative X-fi, and Creative did finally release some drivers for the card to work in Vista. But I never really got over how pissed I was that I could not use any features of the card for so long. When they finally did release some drivers, it did help some, most of the features of the card would finally work. But not all of them. I yanked mine out, gave it to someone, and gave Creative the middle finger up.
You will just have to try both and compare, see what you can and cannot do with the Creative piece of s**t in Windows 7, and make up your own mind. New motherboard though, I am going to say you may do exactly what most people have already done and just forget about add in cards.