I've gone from onboard, to a xonar ds to a creative x-fi in the last couple of months.
On XP i spent a lot on an audigy 2zs, which was useless on vista, and returned to onboard.
Whilst you can have 7.1 onboard sound it doesn't offer a lot in terms of functionality, the cheapest entry level card (£40) will be a huge improvement in terms of versatility and sound quality.
An even better card (£70) will offer even greater functionality (upmix from stereo to pseudo-surround, full eax in games, crystalizer on x-fi cards) and better FPS.
if you are a perfectionist (or just picky) you wouldn't want to step down from a sound card to onboard.