We have a couple threads on crawlspace insulation:
http://ecorenovator.org/forum/renova...psulation.html
http://ecorenovator.org/forum/renova...-flooding.html
With your dry summers and humid winters I wouldn't worry about it horribly much. Doing just the floor and a bit up the wall would probably be enough. However, if it was me I'd probably just go all the way up the wall. Its not that much farther to go, and since you're down there anyways you might as well just do it so you never have to worry about it again. Depending on the age of the house, I'd think the sill plate should have a barrier of some sort under it so its protected from wicking water. Then again my house is over 100 years old, has a very moist basement and to my knowledge the sill plate is fine...