howdy doody. the smbus (system management bus) drivers are part of the unified driver for the nforce and nforce 2 chipset. I found a link to the latest here.
But beware, it warns to use the audio drivers supplied with your board, because for some reason, the new ones will not install. But, there should be no problems with the rest of the driver set. Just read through the article, seems to be a pretty good one.