Check out details about my server here and here. In the last link you'll have to dig deep for what I have done to improve the server but it is relevant to path I had to take to do what I had to do to ensure a good, stable server.

Your issue with 400 players or characters is not an issue, it's the 24 player limit that will make things difficult. The first link above shows my system but in that thread it also shows JoeBoomz' opinions of his ideal system. Other less obvious things are: need no hotshot video card, no big HDD, no other programs except admin tools, adequate cooling even if it doesn't look like it needs it, a KVM switch then you wouldn't need another keyboard-mouse-monitor just for the server. Bottom line, to handle 24 players, be 24/7 (I assume) and be stable look to spend more than $500. Hope that's cheap enough. Just remember cost [uusually[/u (AMD vs intel excluded {Joe }) equates to quality and you are considering a 24/7 uptime, yes?

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Earendil
SysAdmin of Boston Freelancer server
See server rules and server news for more info.