From Windows Server 2000 to ...?

Hi.
I got a very old server, Pentium 3 I believe.
And it has Windows Server 2000, which is connected to 24 client PCs.

Now I want to get a better server, like Pentium 4 atleast with more ram and hard drive space, and also put on it newer Windows Server OC.

Is this reasonable choice, since its server will be faster the OC will consume more resources and therefore be same at performance as the old server, or no?
What kind of Windows Server OC versions there are, beyond 2000?

Thx

Server 2008 is the latest version. It is also available as Small Business Server 2008.

Server 2003 R2 and SBS 2003 R2 are still available also.

What functions did you need from the server (SQL Server, Exchange Server, etc.)?

Cybercafe like, just runs administrative software that monitors clients.
Also thinking of runnig a small cs 1.6 lan server, so people can join and download maps and then host themselves, or maybe even play on the server itself.