Internet Infrastructure

Hmmm. I may be speaking out of turn here, but here goes:

If your apartment building is going to be wired - that is, an ethernet connection in every unit, then your biggest problem is going to be overcoming the 300' maximum length of ethernet circuits. There are ways around it, like adding a hub, or a repeater between floors.
Aside from the physical wiring hurdles, you're going to need an 'access point'. If you have a T1 or DSL or even a cable modem, you can plug that connection into a relatively inexpensive box and install a firewall/proxy solution. The box will need two network cards - one to the Internet connection and one for your LAN (or AAN - apartment area network!). The LAN connection would go to a hub with enough ports for your units/hubs/repeaters. All traffic would get funneled through your one connection to the Internet.

As for the firewall/proxy - there are 'free' Linux solutions - or you could go the Microsoft route and install Windows 2000 Server and use their Internet Security and Acceleration (ISA) server. Either way, with a firewall, you will always have issues with users wanting to play games or use programs that require you to 'open up' additional ports. Keep that in mind when choosing your firewall solution, as maintenance and monitoring are going to be continued activities.

Hope this helps!
-Mike

The first answer was pretty good, just a few extra remarks...
Use switches, no hubs because hubs can slow down all computers on the network, switches will provide the users with a much better surfing experience, not to mention online gamers that will not like the collision delays of a hub!
A Linux box with firewall, proxy, NAT and perhaps dhcp-forwarding might be the best shot, so the users can choose if they want to use the NAT and proxy and in turn may be not able to use this game or file exchange program, or to go for the dhcp forwarding with all the risks a permananet internet connection has. If you want to provide good, reliable service at reasonable cost you should make NAT and proxy mandatory. That way you can eliminate the problem of single users using up all bandwidth e.g. with a fileshare, mp3 server, etc... at the same time you provide extra security, and less things can go wrong, you only need one IP from your Provider what is going to be cheaper.... and it is amazing how much more users can comfortably share the same line when going thru an well configured linux firewall/proxy server than thru the direct connection. Especially under higher loads, the unregulated traffic will just colaps while the proxy will only slow down when the line gets full.