You won't need gigabit for the games.
Games like SoF2 and CoD use about 3 - 4 KBps for each user for download at the server. Upload to the server per user is less. I doubt any games you play will use much more than 5 KBps per user.
So if I run a server for 32 people, thats maybe 160KBps, or 1280kbps. And if your network is 100mbps, then the games will only use about 1.3% of your network bandwidth at the server for download, and probably 1% or less for uploads at the server.
As for the game server speed, I've noticed on mine that the only time the server really does any work is when changing maps. If you're going to run multiple game servers at a time, I would suggest with that many users that you have a different box for each game server instance.
For reference, I ran SoF2 on a AMD XP1600+ with 512mb PC2100 ram and 20gb 7200rpm drive and WinXP Pro. It ran perfect with about 20 users, loaded levels very fast, and in general worked perfect.
If you're worried about bandwidth during the lan party, you may want to run Bandwidth Monitor Pro on your servers. You can get a 30day trial for free. WinXP has a network bandwidth tool built in also.