I think that the engine used in your game is multithreaded, wich means that the game code has two or more threads (threads being the basic cpu food) that can be run at the same time. This will (well, sometimes) benefit the game if run on a single cpu system, but will be an even greater benefit on a smp (Symetric Multi Processor) system. Some game server apps don't like to run on smp systems. In this case it's settled, run a single cpu system... The smp sys can run both threads at the exact same time. Usualy the game divides networking and physics into two separate threads. The server will put all this together and send it out as gamestate to the clients.
With this being the case in more and more games; UT and any of its progeny, Q3 and its derivatives use multithreading, running a dual cpu system is a good idea.
I would do a AMD system over an Intel, because the floating point performance of the AMD offerings is typicaly better than Intel on the p4/xeon cpu. If your game doesn't use sse2 much, the AMD is better on average. A "dual core" system will have the same operating characteristics as a true smp system, far as the game server is concerned.
Fastest cpu and most memory the mobo will take that you can afford is normaly the way to best server performance. So it's primarily a money issue.
For some idea as to what parts are providing good service to the game, play it. Look at the hardware specs of the servers you play on if you can, and compare them. Talk to admins and see what they have found to perform well. Then purchase accordingly. There is a cost to performance ideal at work here, no sense in buying 4 gigs of ram when the game will only use 250 megs or less, on top of say 256 (ok maybe 150) for the os itself. In this case 512 megs total would do fine. If you happen to run a game that takes 500 or more, a gig plus would be a good choice. If you want to run multiple instances of the same server, or different game servers concurrently on the same box, more memory is always best. Or forget all this and run what you have!
As far as software needed, the game will include dedicated server apps, and sometimes you can dl a stand alone server app. There are normaly 3rd party admin utilities to make administration "easier", but I don't like these. I prefer setting up the bandwidth used and player count, etc, as a text file usualy found within the game's config files.
There are some hardware and software tuning tricks I have found to be worthwhile that I can let on, if the mods will make it a sticky. A lot of the time they can be used to improve performance on a gameing system as well as a game server. The software tips are for M$ operating systems, although Linux can benefit from the hardware tips and the software tips can be alegoricaly implemented on a nix sys.
Thus endeth the sermon.
