You shouldn't have any problems with plugging all the machines into one hub.
A switch isn't necessarily faster than a hub. The logic built into a switch introduces some latency.
In any case i've had LAN parties with 8 to 10 people plugged into a single 12 port. We didn't have a dedicated server though, so I don't know how that will play into things.
Edit: I just wanted to say that I've had nothing but problems playing HL2 DM LAN. There are plenty of write ups about it, but I can't get it to work anyway.
About the GB switches, they are nice but everyone will have to have GB nics. The ports on a GB switch pipe to match the nic plugged into it. So a 1000mbps port when plugged into a 100mbps nic becomes a 100mbps port. Does that make sense?
Hope this helps.