[Scott L.]

Is this a school inviroment, lan party attendees usually need to grab the latest update or patch, and most school networks block these sites. If this is the case find out what games will be played and download the patches etc into a common folder shared over the network and let everyone know where they're located.
40 attendees is no problem, just use the advice given. so if 20 people required 2 circuits, than 40 would require 4 circuits. Also don't forget the power requirements of the network equipment like routers, switches, servers, etc.
also make sure you have access to the breaker box in case a circuit should trip. Static IPs are good if no router is used, people signin and get the ip next to were they sign. Dynamic IPs make it easier sometimes, IP is assigned by a DHCP server/router.
I haev experience running a LAN party so if you need help send me a message.