ive run lan parties in excess of 200 people
what works for us in regards to power (and power is almost always an issue event hough i usaully outsource the entire power thing to a party rental place) is that you can place 4-5 PC's per 15amp breaker, that only being their TOWER and MONITOR, nothing else, no mini TV's, no 500 watt klipsch speakers, no microwaves (yes ive had people bring microwaves??? WHY?!?!) no christmas lights strung up around your gaming area, cuz every watt counts when you start dealing with 40-50+ people
Remeber, all teh outlets by your kitchen/bathroom are 30 amp breakers, and if you rEALLY need power, buy a distrubtion board (couple hundred bucks) and plug it into the back of your oven or laundry dryer outlet