Does Steam store my savegames?
If I install my steam on another computer, will I start my games from where I last saved them?
In my experience, no. You'll need to move over your saved games manually, and how that's done changes from one game to another.
Also, I believe Steam is supposed to keep track of achievements, but it's been hit or miss for me. I kept 2 (out of about 6 or 7) achievements for Portal, but kept all of them for Braid.