I have a signed copy of Foundation that is my proudest geek possession... saw him speak twice. I'd second the Lije Bailey/Daneel Olivaw books for a good start -- start with The Caves of Steel and The Naked Sun.
Two of my best "geek possessions" are the last couple of parts to both "Currents Of Space" and "Now You Don't" in their original magazine forms.
As for the Foundation books, I only read the original trilogy when I re-read the books. Don't care for the books written from the 80's up. Got tired of going back to Seldon, who really only serves the purpose of setting the whole thing up.
These books work almost like a stage play and I love all the conversations about history and psycho-history.