I'd say you could easily run it on a 200MHz box. It wouldn't actually be usable for large numbers of players though. It will also depend on what else you want the box to do? InSim apps? Other daemons/services? What OS? Specifically these last few would make the world of difference.
I'd be surprised if anyone has any functional hardware floating about less than 400MHz these days...
You need to bear in mind that system resources generally don't hold the dedicated server back. Bandwidth and transfer does.
Do you have upload speed and bandwidth? Those things will cripple you waaaaaaay more than hardware specs. Hell I can run a server on my 7-year-old laptop that isn't much more powerful than the specs you quoted but my upload strength is only sufficient for 6-8 players.
PII 350mhz, 320Mb RAM running FreeBSD made for a great server before we got "proper" dedicated servers for our team
Actually, that PII acts as my router / firewall these days.
Downstairs though, I have a PI 75Mhz (was originally a 486DX) with 20Mb RAM.. still runs, although not much use for the most part.. but it's loaded up with win3.1, m$ office 4.2 and other such "oldies" :cool: