If you didn't do anything other than the cap and rotor at this time, it's unlikely they were responsible for the smoke. Was the engine cold when you started it? If so, it is likely that the smoke has always been there on cold starts (probably due to hardened 21 year old valve guide seals) and you just haven't noticed it before.
Unless your plugs were really old and fouled, it's unlikely that the hotter spark would have burned enough gunk off of them to be noticeable coming out the exhaust. Where you WILL notice the hotter spark is in easier starting, smoother idle and acceleration, and better gas mileage.
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