I can't agree more with the comment about knowing your house's circuits. Figure that everyone will have a monitor and a tower, and possibly speakers (though headphones
are preferred). If you've got 15 people, you've got about 30 devices to plug in, all of which are gonna be sucking down a
lot of power.
As for monitors - yea, we normally bring our own, which makes lugging CRTs not fun, but there's nothing you can do. Unless you run "professional" lan parties, and charge people a lot, it's not even remotely practical for the host to supply monitors.
The way we do LAN parties: The host supplies the house, the guests bring themselves, their computers, food, and alcohol.