minimum safe distance from light bulb to drywall?

???? I doubt you'll find any sort of authorative answer for your question because buliding codes simply don't address this sort of thing.

Electrical codes will adreess the types of light fixtures that are allowed to be installed in specific situations and the UL rating on the fixtures will determine the maximum wattage of bulbs used in them.

The type of bulb used would also affect how it disburses heat.

fishin is correct. If the fixture is rated for 200w then it should be fine.

Unless you are doing on odd installation that puts the fixture right up against the wall you should have no problem. I'm not sure why you would want 200w in a fixture that close to drywall. Nor can I think of any standard household fixture that would normally need 200w. That's a lot of lumens from one source.

You might want to consider one of the new high watt compact flourescents. They come in 150w equivalents and may come larger. They reduce the wattage and have less heat allowing you to put more lumens into a fixture rated for 100w.