Personaly i would go with the sb live. my last card was one of those. It was a lower end model, it would out perform my onboard in every way, especialy games. Dont get me wrong its not going to make a huge difference, but a little bit here and there can make all the difference. The sb's are outdated, fact, but still it is a dedicated card and will work to the most part on its own reasources and will only use system resources when being pushed, where as your onboard no matter how good a quality, will take ALL its resources from your system!
Remember to get the latest drivers for it though!
If you decide you dont like it you can always go back, no harm done!