Bump from the grave...

I have to change out the sparkplugs in my car soon. I think I'm going to go with the platinums, but how much of a longevity increase do they offer over copper? How long before you have to change out copper plugs, and how long is it before you have to change out platinum plugs?

And btw, where is the best place to buy platinum plugs? Can I install them myself? Do I need a torque wrench? Do I have to manually set the gap?