I lol'd at OP. As long as you do the Upgrade version, and not a "Fresh Install" you shouldnt lose your tuff, but it would be wise to run Time Machine before you back up your stuff.
I love the thing how spotlight will search faster in SLeopard and how Time Machine will backup faster, but-
Will the Time Machine saves in Leopard work in Snow Leopard? (Backward Time Machine save compatibility?)