What advantage would a server OS get him. No server based apps are bing installed (Ms sql, iis or exchange)
If you are running into tcp-ip limits you can remove that. Rather then buying server 2003 or 2008 ror $600-800, i would run the patch from this website
You will not be able to have 2000 users on a single server. These online games consist of dozens of game servers, storage, and sql servers, fast routers and network connections.
If you want to play online with 20 or so friends perhaps you will be ok
The home router you have will not be able to handle a large number of connections.