Things That Make You Go Sigh

I noticed yesterday that some of the planks on the deck out back are beginning to rot out. The deck is many years old with a rather nasty spiral staircase down to the patio outside the basement and has been deteriorating steadily despite applications of sealant and the like since we moved in six years ago.

The Missus and I have had a standing joke about waiting long enough to have a blizzard or a hurricane knock the thing down and then letting the insurance company build us a new one, but alas, it looks as if I'm going to have to start investing in two by six lumber myself. (At least the pillars and beams on which it is built seem to be sturdy enough for now.) The question is whether I should just replace individual boards as they become unsound or whether I should bite the bullet and redo the whole thing at once. I mean, it's not like I've got anything else to do with myself, right?

UPDATE: Oh, and don't bother telling me about that new non-wood decking material. Our neighbors have a deck made of it and, frankly, I don't like it very much.