I have at least a dozen 4GB CF cards that have been sitting in the drawer since I sold my D300 last year. I'm hoping that I can use them in the D400.
I'm a little nervous about the SD cards (use them in D7000 and V1 cameras), because they are quite delicate and need to be handled carefully. But like you, I like the dual-slot capability, which means I won't have to change them out too often. bigbear1