I take it you want to do this correctly, am I right? Then let's start with some basics.
First off, your client machines should be (to stick with Windows products), either Win2K Professional or WinXP Professional. WinME has no real security functionality, hotfix management, or stability. My preference would be to use Win2K, but if these machines will also be doing some gaming, then you will need the power that XP provided. Basically, XP has the stability of the NT kernel with the gaming powerof Win98. Win2K is not a good gaming platform. It is stable, yes; it is secure, yes; but there can be gaming problems.
So, XP would probably be the best bet for the client workstations. YOu can set up each client machine to auto-login with the same domain user name that has limited rights, while many system features are locked down. There are many articles out there detailing how to set this up.
Now, as for the server question, I suggest you run 2 servers, for one simple reason: if a server dies, you still have the other one up! If you run your Internet gateway and central game server on the same machine, and if the NIC on this server goes out, then your entire buisness is offline! And downtimes like that are not usually acceptablefor any business owner! This is called "distributed computing", and it is easy to grow from this method also.
Hence the name of my comment. It is easier to scale up to more machines and more content provided with distributed computing. And the more content you can provide in a secure and stable environment, then the more business you can create. And the more revenue coming in.
Good luck