CA12 runs from Sonoma to Santa Rosa to Sebastopol. Looks like its mostly 4 lane or divided, and probably busy. Calistoga is over a ridge from Santa Rosa, and 12 does not go anywhere near. The road(s) over that ridge may be curvier than you like.
As stated, hwy 12 doesn't go to Calistoga. The Petrified Forest/Mark West Springs road between Calistoga and northern Santa Rosa is ok but not Calistoga road into eastern Santa Rosa.