Unreal Tournament 2004 server will run on a:
Pentium 2 300MHz / 64MB (96MB WindowsXP) / 250Kbit Broadband Connection
infact really *the* most important aspect of an internet server is the connection. remember that server-side machines should not be designed to play the game, but dedicated data transfering.
the only reason to have a quicker CPU, and Ram is to get a quicker throughput so you can effectively use a quicker connection.
Remember almost everything to do with these games which takes processor speed; Physics, Graphics, Sound, Networking take up ALOT of power. A Server-Side machine only has to concentrate on storing and moving data; it doesn't need physics, graphics, etc...
Connection however is the most important aspect.
While my 300Kbit connection is quite happy serving a UT2004 game (non-dedicated) with upto 6 people, any more than 4 and pings will go from 30ms to 180ms
The job of a server is to make sure that pings stay as low as possible. Because of how home connections are designed, you will find that a T1 line although only capable of 1Mbit data throughput is capable of handling without problem upto 16 similtaneous connections while your 3Mbit ADSL will choke after 4.
Speed is not the most important aspect for servers, learning this will help you put together a system which can handle as many people as you need.
Dial-up :: Max 3
xDSL :: Max 4-6
Cable :: Max 10 (however half must be on-site)
Terminal 1 :: 16
Terminal 3 :: 128
Optical :: 65535
You will have to take into account the processing power required for each person as well though. Each player in UT2004 will need roughly 10MHz Processing power and 8MB Ram. So try to remember this; especially when putting together Server Farms; To calculate how much power each person will take you basically find out how fast the ping call back is for one person is, then calculate based on processor speed how fast it is going.
Although it wouldn't be a literal 10MHz, so thinking you can handle 1 person on a 286 or something; it is more a way of figuring out how many resources they take on a high end system.