Shane-
as others said, LOTS of power. but it also depends on atmospheric factors. there is a member of this forum that does shows professionally that just did a show with (2) 30-40 Watt Laser Scopes and he said the visibility sucked. it was an outdoor show and the atmosphere just did not cooperate with him. The next night, they used 10 fog machines and got a much better show.
however, on average, with average atmospheric conditions with average humidity, i would say start with a minimum of 2-3 Watts of green (532nm). and you are correct, the "slower" type of beam type effects will provide much better visibility also. Stick with "Finger" type fans. individual effects where there are less points and therefore more beam visibility.
If you project a "liquid sky" ( a solid sheet ) above the crowd, it is going to be very hard to see it in adverse conditions. However, if you slow that sheet down greatly, you will begin to see individual beams and the effect will become more crowd pleasing.
also, as i am sure you are aware, *NO* beams can ever go into the crowd. (there are exceptions to this, but it is not worth looking into at this point in your laserist career).
You came to the right place to soak up as much information as needed. its good to see you are looking to educate yourself PRIOR to doing shows. Feel free to ask as many questions as needed. Whether it be for variance issues, equipment questions or just plain advice. we're all (for the most part) cool guys and gals willing to help.
Its good to have you here. Try to read up on the SELEM meet coming up also. Its a weekend of a bunch of us laser geeks gettin together and just having a blast!! youll see some pretty SERIOUS lasers from RGB lasers to Multi-Watt greens to 8-10+ Watt ion systems to some very influential indistry pros coming and hanging out with us! Its definintely worth the trip.
Stick around and soak up the knowledge.
-Marc