enough hard drive space for installing games or have them pre-installed, estimated space for games installed: 5-10GB
Monitor
A decent monitor is a must for adequate play. You will be in front of the computer screen for a while, so a high refresh rate is a plus. Small monitors are also godsend when transporting from location to location, anything over 17" is not fun to carry around.
Keyboard
A no frills keyboard will do just fine :)
Mouse
Any standard mouse will do just fine, but it is recommended you take one that is clean and are comfortable with. Most games played at the LAN party will require the use of a mouse, and falling behind in score due to an unresponsive or dirty mouse can be frustrating. A mouse pad is optional, but recommended, as the surfaces used for mouse tracking will be less than adequate for many precise movements required by some games.
Sound Reception Device
Headphones are highly recommended. I can't stress the important of a good pair of headphones. If you choose, you may bring speakers instead. There are a few inherent problems with speakers, however. One is noise pollution, which degrades audio quality and makes it hard to listen to your game environment. In some games, this is a crucial part of being able to play. They're also bulky and a pain to carry.
Cables
The following cables are absolutely necessary for providing power to your equipment as well as connecting to any hubs present at the LAN for network access.