Doing a quick analysis,
Let's assume 200 watts for a PC:
100 Watt processor, 100 watt Video card, the rest is inconsequential. Being as it's a LAN party, assume everyones playing hard so the processor and video card are using max power.
Using sao123's numbers for a monitor, that gives us about 275-350 watts per PC, or 2-3 amps.
Multiplying by 80 computers, we're talking 160-240 amps, not taking into account 80 ports worth of ethernet switches and the stereo system that's cranking the tunes, the lights, and the air conditioning system to keep 25KW of heat from turning you all into parboiled pot roast.
In the US, it is extremely common for a new single-family house to have 200 amp service. Older houses (say, 1970's and earlier) had less - perhaps 100 amps. Even older might have had 60 or less. 200 amp service means pretty much what you think it means - that's just about all your house can pull from the power company. So, you're probably only going to be able to plug in half your computers at one time, and you're only going to be able to plug in 7-10 computers to each outlet circuit in the house.
Maybe you need to borrow all your neighbors outlets too?
/frank