Without a lot more information, it is hard to be specific.

First - it might be easier to split the existing lighting and use two dimmers - it depends on how the house light circuits are wired. One theater has four separate house light circuits, each ison a different dimmer. They are able to make very interesting transitions as the house lights go to half then out.

The next issue is the height of the existing fixtured because that will limit the selection of lamps. Years ago, I worked at a theater that had 500 watt mogul lamps to light the house - I think that just two or four fixtures covered the entire house. If you just reduce lamp wattage the house may seem dark.

In general, if you can move to a halogen fixture you can get the same lumens out of a smaller lamp - we use Par 38's for house lighting - a 90 watt halogen Par 38 has the same output as a 150 watt standard incandesant Par 38.

I know I don't have to say it but - don't use compact flourescent lights - they don't work on dimmers - even the ones that advertise that they work on dimmers.